Bid‘ah is the every introduced innovation that did not exist at the time of the Prophet Muhammad ﷺand his companions رحمت الله. Moreover, Bid‘ah refers to every addition or deletion made to the religion after the age of the Companions that is not approved by the Sharee‘ah (Islamic Law). From the point of view of Shari'ah, every bid’ah is Zalalah and there is no such thing as 'good' bid’ah. Prophet ﷺsaid in a hadith, "Kullo bidda tin dhalala (every bid’ah is Zalalah )." Zalalah means to be astray or to be away from the truth.
The sinner, at least, knows that whatever he is doing is wrong and doesn't claim that it's halal and should be done to please Allah سبحانه و تعالىand so on. However, the one who commits bid’ah not only goes against Qur’an and Sunnah(Saying and Actions of Prophet Muhammad ﷺ) but also claims that what he does is pleasing to Allah سبحانه و تعالىand a way of getting closer to Him. 
The word Bid’ah (Arabic: بدعة) or "innovation" linguistically means anything new or newly invented matter'.
Bid’ah is any type of innovation in Islam. A Bid’ah is anything that has no basis in the Qur’an, Sunnah or sayings of any of the companions.Though innovations in worldly matters, such as science, medicine and technology are acceptable and encouraged, Bid‘ah within the religion is considered a sin.
Imaam Ash-Shaafi’I رحمت الله said: “A Bid’ah is anything that has no basis in the Qur’aan, Sunnah or sayings of any of the companions رضي الله عنهم.”
Ibn Al-Jawzi رحمت اللهsaid: “A Bid’ah is any form of worship that did not exist (at the time of the Prophet ﷺand his companions رضي الله عنهمthen later it was innovated.”
Ibn Rajab رحمت اللهsaid: “A Bid’ah is any form of worship which has no basis in the Sharee’ah (Islamic Law).”
The Messenger of Allah ﷺsaid: "Every innovation is misguidance and going astray" Reported by Sahih Al- Bhukari H:2697. Abu Daawood H:4607, At-Tirmidhee H:2676) and it is saheeh. Ibn Hajr رحمت اللهauthenticated it Takhreej Ahaadeeth Ibn ul-Haajib 1/137.
And He ﷺalso said: "... and every innovation is misguidance and all misguidance is in the Hellfire." Reported by An-Nasaa'ee H:1/224 from Jaabir bin Abdullaah رضي الله عنهand it is saheeh as declared by Shaikh ul-Islaam Ibn Taymiyyah رحمت اللهin Majmoo' ul- Fataawaa 3/58.
The Messenger ﷺalso warned against the People of Innovation, from befriending, supporting or taking from them saying: "Whoever innovates or accommodates an innovator then upon him is the curse of Allah سبحانه و تعالى, His Angels and the whole of mankind." Reported by Sahih Al-Bukhari H:12/41 and Sahih Al-Muslim H:9/140.
Prophet Muhammad ﷺsaid, “…and the best speech is the Speech of Allah سبحانه و تعالى, and the best guidance is the guidance of Prophet Muhammad ﷺand the worst of all affairs are the newly invented matters (in the religion)” Sahih Al-Muslim H:867.
Prophet Muhammad ﷺsays in this respect: “He who innovates something that is not in agreement with our matter(religion), will have it rejected.” Sahih Al-Bukhari 2697 and Sahih Al-Muslim 1718
Narrated Anas رضي الله عنه: The Prophet ﷺsaid, "Some of my companions will come to me at my Lake Fount, and after I recognize them, they will then be taken away from me, whereupon I will say, 'My companions رضي الله عنه!' Then it will be said, 'You do not know what they innovated (new things) in the religion after you." Sahih al-Bukhari 6582 (Vol. 8:584)
Every act of worship that is induced by people and was not ordained or approved by the Messenger ﷺis considered an act of Bid`ah, for the Messenger ﷺsaid: Whoever introduces a practice which is not authenticated by me, it is to be rejected
Therefore, Muslims should adhere to Islam and abide by it, for this will suffice them and lead them to attain perfection (of Eeman), for they are not in need of such acts of Bid`ah, Allah سبحانه و تعالىsays: This day, I have perfected your religion for you, completed My Favour upon you, and have chosen for you Islâm as your religion. Qur’an.Surah Maidah 5:3.
Allahسبحانه و تعالىhas completed the Deen (way of Life as prescribed by Allah سبحانه و تعالى) and made it perfect by virtue of His Grace. Thus, people do not need to observe acts of Bid`ah. The Prophet ﷺalso said: You must adhere to my Sunnah and the way of the Rightly-Guided Caliphs, who come after me. Adhere and cling to it strongly.
People are in no need of acts of Bid`ah introduced by so and so; rather, they should adhere to the Prescriptions of Allah سبحانه و تعالى, follow His Path, avoid His Prohibitions, and abandon whatever is innovated by people (in religion). Allah سبحانه و تعالىhas described the acts of Bid`ah and their people in His Saying:Qur’an.Surah Shura 42:21 Or have they partners (with Allâh سبحانه و تعالى— false gods) who have instituted for them a religion which Allah سبحانه و تعالىhas not ordained?
Allah سبحانه و تعالىSays (what means): “On the Day [some] faces will turn white and [some] faces will turn black, [to them it will be said]: 'Did you disbelieve [i.e., reject faith] after your belief? Then taste the punishment for what you used to reject.” Qur’an.Surah Al-Imran 3:106.
In tafseer of above ayath Ibn ‘Abbaas رضي الله عنهcommented: "The faces of the people who adhered to the Sunnah will turn white and bright, while the faces of the innovators will turn black and gloomy."
Allah سبحانه و تعالىSays (what means): “And whoever opposes the Messenger ﷺafter guidance has become clear to him and follows other than the way of the believers – We will give him what he has taken [i.e., make him responsible for his choice] and drive him into Hell, and evil it is as a destination.” Qur’an.Surah An-Nisa 4:115.
Allah سبحانه و تعالىSays (what means): “And [moreover], this is my path. Which is straight, so follow it; and do not follow [other] ways, for you will be separated from His way. This has He instructed you so that you may become righteous.” Qur’an.Surah Al-Anaam 6:153. Mujaahid رحمت اللهcommended: "The other ways mentioned are desires and innovations."
1. The act of worship must coincide with the Sharee’ah in the reason for which it is done, or else it would be rejected.
2. The act of worship must coincide with the Sharee’ah in its category. Therefore, it is not acceptable if a person were to slaughter a horse instead of a camel or goat as a sacrifice after Hajj.
3. The act of worship must coincide with the Sharee’ah its quantity. Therefore, if somebody were to add two Rak’ah to the Dhuhr prayer, making a total of six Rak'ah instead of four, this would be
unacceptable as it would not be coinciding with the Sharee’ah in its quantity.
4. The act of worship must coincide with the Sharee’ah in the manner or order in which it is performed. Therefore, if someone were to begin their wudoo' by washing their feet instead of their
hands, then this act would be unacceptable.
5. The act of worship must coincide with the Sharee’ah in the timing of its performance. Therefore, praying the Dhuhr prayer after sunset is unacceptable.
Al-Hasan al-Basri رضي الله عنهsaid, "Do not sit with the people of innovation. Do not debate with them or listen to them." Ad-Darimi H:1/121.
Abul 'aliyah رضي الله عنهsaid, "Learn Islam. Then when you have learned Islam, do not turn away from it to the right or to the left, but be upon the Straight Path and be upon the Sunnah of your Prophet ﷺand that which his Companions رضي الله عنهمwere upon ... and beware of these innovations because they cause enmity and hatred amongst you, but stick to the original state of affairs that was there before they divided." Al-Hilya of Abu Nuaym, 2/218.
Sufyan ath-Thawri رضي الله عنهsaid, "Innovations are more beloved to Shaytan than sin, since a sin may be repented from but an innovation is not repented from." Sharh Usul I'tiqad of al-Lalika'i (d. 414H) H:238, the great 4th century Imam who wrote on the belief of Ahl us-Sunnah] This is because the innovator believes he is doing something good and therefore sees no need to repent. He also said, " ... so cling to the original state of affairs," Al-Hilya, 6/376. meaning the way of the Prophet ﷺand his companions.
Imamal-Awza'i رضي الله عنهsaid, "Patiently restrict yourselves to the Sunnah, and pause where the people paused, say what they said, and avoid what they avoided. Take to the path of the Pious Predecessor, for indeed, what was sufficient for them is sufficient for you." Ash-Shari'ah, p58.
Ibrahim al-Masayrah رضي الله عنهsaid, "He who honours an innovator has assisted in the demolition of Islam." Sharh Usul I'tiqad, 1/139.
Imam Abu Hanifah رحمت اللهsaid, "Stick to the narrations and the way of the Pious Predecessor, and beware of newly-invented matters for all of it is innovation." Sawn ul-Mantaq of as-Suyuti, p32
Imam Malik رحمت اللهsaid, "He who innovates an innovation in Islam regarding it as something good, has claimed that Muhammad ﷺhas betrayed his trust to deliver the message as Allah سبحانه و تعالى says, "This day I have perfected for you your religion." And whatsoever was not part of the religion then, is not part of the religion today." [Al-I'tisam] He also said, "How evil are the people of innovation, we do not give them salam." Al-Ibanah of Ibn Battah (d. 387H), Page. 441.
Imam ash-Shafi'I رحمت اللهsaid, "Whosoever considers an innovation to be good has corrected the Prophet ﷺ." [Bulugh al- Maram of Ibn Hajar رحمت الله(available in English), p190 footnote 2] And all that has been related from him that contradicts this statement, regarding innovation as being of two types is either weak or baseless as al-Hafidh adh-Dhahabi رحمت اللهand others from the scholars of hadith state.
Imam Ahmad رحمت اللهsaid, "The fundmental principles of the Sunnah with us are: ... avoiding innovation and every innovation is misguidance." Usul us- Sunnah (available in English), p.1
Imam al-Bukhari رحمت اللهsaid, "I have met more than a thousand scholars ... (then he named the more prominent in each of the lands he visited) and I found that they all agreed on the following points: ... they all used to prohibit bid'ah - that which the Prophet ﷺand his Companions رضي الله عنهمwere not upon, because of the saying of Allah سبحانه و تعالى, '...and hold fast to the rope of Allah سبحانه و تعالىand do not separate.' " [From his article on belief, quoted inSharh Usul I'tiqad, 1/170. Amongst the scholars he met were Ahmad bin Hanbal رحمت الله, Abu Ubayd al-Qasim رحمت الله, ibn Ma'in رحمت الله, ibn Asim رحمت الله, ibn Abi Shaybah رحمت الله, and Abu Salih رحمت اللهthe scribe of Layth bin Sa'ad رحمت الله.
What we have narrated to you here is a small number of narrations from amongst literally hundreds from the early scholars of Islam with regard to the sin of innovation. 
Ibn ‘Abbaas رضي الله عنهwas circumambulating the Ka’bah with Mu’aawiyah Ibn Abu Sufyaan رضي الله عنهduring Mu’aawiyah’s رضي الله عنهreign. Mu’aawiyah رضي الله عنهwould touch all four corners of the Ka’bah while circumambulating and so Ibn ‘Abbaas رضي الله عنهremarked: "The Prophet ﷺonly touched two corners (the black stone and the Yemeni corner)." Mu’aawiyah رضي الله عنهreplied: "No part of the house of Allah سبحانه و تعالىshould be neglected." Thereupon, Ibn ‘Abbaas رضي الله عنهrecited the ayath (which means): “There has certainly been for you in the Messenger of Allah ﷺan excellent pattern (i.e., example to be followed) for anyone whose hope is in Allah سبحانه و تعالىand the Last Day and [who] remembers Allah سبحانه و تعالىoften.” Qur’an.Surah Al-Ahzab 33:21. Upon hearing this, Mu’aawiyah رضي الله عنهrefrained. Tirmidhi H:858