When Prophet Muhammad ﷺ and his companions رضي الله عنهم left Makkah for Medina in what is known as Hijra. They were called Muhajirun. The Arabic root word for immigration and emigration is Hijrath. The counting of Islamic calendar starts from this Hijrath
About six months after the Muslims of Makkah had left their homes, livelihoods and in many cases, their families, for the sake of Allah سبحانه و تعالىand migrated to Madinah, the Prophet Muhammad ﷺestablished a system that would bring together Muslims of two different cities and different tribes, races and ethnic backgrounds. Enter the Muslims of Madinah. They became the Ansars (helpers) of their Muhajir brothers and sisters in faith. 
The Muslims who had left Makkah were called Muhajirs, migrants who had left their homes for the sake of Allah سبحانه و تعالى. Or A Muslim who emigrated from Makkah to Madinah during the time of the great repression of the Muslim by the pagan Quraish up until the conquest of Makkah 8AH by the prophet Muhammad ﷺ. Its Plural is Muhajireen. 
And the first forerunners [in the faith] among the Muhajireen and the Ansar and those who followed them with good conduct – Allah is pleased with them and they are pleased with Him, and He has prepared for them gardens beneath which rivers flow, wherein they will abide forever. That is the great attainment. Qur’an.Surah Tauba 9:100.
So if they believe in the same as you believe in, then they have been [rightly] guided; Qur’an.Surah Al-Baqra 2:137.
"Indeed, Allah was pleased with the believers when they gave their pledge to you (O Muhammad) under the tree. He knew what was in their hearts and He sent down calmness and tranquility upon them..." Qur’an.Surah Al-Fatha 48:18.
"Among the believers are men who have been true to their covenant with Allah and showed not their backs to the disbelievers, of them some have fulfilled their obligations and some of them are still waiting, but they never changed (i.e. they never proved treacherous to their covenant which they concluded with Allah ) in the least." Qur’an.Surah Al-Ahzab 33:23.
Narrated Abu Sa`id رضي الله عنه: The Prophet ﷺsaid, "Do not abuse my companions for if any one of you spent gold equal to Uhud (in Allah's سبحانه و تعالىCause) it would not be equal to a Mud or even a half Mud spent by one of them." Sahih Al-Bukhari 3673 (Vol. 5:22) and Jami Tirmidhi Vol. 1 : 3861 
Narrated `Abdullah رضي الله عنه: The Prophet ﷺsaid, "The best people are those of my generation, and then those who will come after them (the next generation), and then those who will come after them (i.e. the next generation), and then after them, there will come people whose witness will precede their oaths, and whose oaths will precede their witness." Sahih Al-Bukhari 6429 (Vol. 8:437) and Sahih Al-Muslim H:6159 
Allah سبحانه و تعالىmentions the types of believers, dividing them into the Muhajirin, who left their homes and estates, emigrating to give support to Allah سبحانه و تعالىand His Messenger ﷺto establish His religion. They gave up their wealth and themselves in this cause. There are also the Ansar, the Muslims of Al-Madinah, who gave asylum to their Muhajirin brethren in their own homes and comforted them with their wealth. They also gave aid to Allah سبحانه و تعالىand His Messenger ﷺby fighting alongside the Muhajirun. The Muhajirun and Al-Ansar are the supporters of each other; Allah سبحانه و تعالىpraised the Muhajirin and the Ansar in several Ayat of His Book and His Messenger ﷺas mentioned in Qur’an .Surah Taubah 9:100, 9:117 
Sufyan ibn `Uyainah رضي الله عنهsaid: "He who speaks a single word against the Companions of Allah's Messenger رضي الله عنهم then he is an innovator."
And Imam Ahmad رحمت اللهsaid: "If you see anyone speaking ill of the Companions عليه السلامof the Messenger of Allah ﷺ, doubt his Islam."
And Adh-Dhahabee رحمت الله said, "Anyone who criticizes them or insults them has gone out of this religion and has segregated himself from the Muslim Ummah. He has disbelieved in what Allah سبحانه و تعالىthe Most High says concerning them in His Book, and what Allah's Messenger ﷺhas said concerning their noble qualities and their merits... It is incumbent upon the Muslims to love Allah سبحانه و تعالى; to love His Messenger ﷺ, to love what the Prophet ﷺbrought as guidance and as his practise; to love his descendants, his Companions, his wives, his children, his servants, and to love those who love them and hate those who hate them, as this is the meaning of loving for Allah's سبحانه و تعالىsake and hating for Allah's سبحانه و تعالىsake, and that is the most excellent kind of faith."
In al-Kaba'ir (The Greatest Sins), we find that adh-Dhahabee رحمت اللهalso said, "The one who loves the Prophet ﷺloves and respects each and all of his Companions رضي الله عنهم. To hate any of them is to hate the Prophet ﷺ."
Al-Fudayl ibn `Iyyad رحمت اللهspoke similarly when he said, "Indeed, I love those whom Allah سبحانه و تعالىloves. They are those from whom the Companions of Muhammad are safe. I hate those whom Allah سبحانه و تعالىhates. They are the people of the deviant sects and innovation."
These statements of the great Imams of the Ummah should not surprise us, because a slight doubt about the Companions رضي الله عنهمwill in fact be accusing the Prophet ﷺhimself of hiding the truth about them which he knew (that they were hypocrites, as heretics claim), but he did not tell us!! Or one is accusing him of being something like an imbecile, as Shaikh Ja'far Idris رحمت اللهonce spoke, because the Prophet ﷺ, supposedly did not know although he lived with these people all the time. He thought that they were the best Muslims, but they were hypocrites, (the heretics say). That is why al-Barbaharee رحمت اللهsaid, "Know that anyone who tries to attack the Companions of Muhammad ) really seeks to attack Muhammad ." 
Shaykh al-Islam Ibn Taymiyah رحمت اللهsaid:
If a person slanders them in a way that does not impugn their good character or religious commitment, such as describing one of them as being stingy or cowardly or lacking in knowledge or not being an ascetic and so on, then he deserves to be rebuked and disciplined, but we do not rule him to be a kaafir because of that. This is how the words of those who were not regarded as kaafirs by the scholars are to be understood.