Lying is the complete opposite of truth. So, anything that is untrue and deliberately intended to mislead another person is a lie. A lie, therefore, can be anything spoken or written that is totally or partially baseless, unreal, made-up, distorted or exaggerated; for example, if someone were to deliberately state that a five foot pole was a ten foot pole, then this would be a lie. Similarly praising someone out of proportion is a form of a lie.
The worst form of lying is to do so upon Allaah and His Messenger(S) which is to falsely attribute things to them. And (what means): "…And do not conceal testimony, for whoever conceals it – his heart is indeed sinful…." Quran Surah Baqrah 2:283
And (what means): "And do not mix the truth with falsehood or conceal the truth while you know [it]." Quran Surah Baqrah 2:42
Hypocrites are liars too, because they lie to themselves. Allaah Says about them (what means): "In their hearts is disease, so Allaah has increased their disease; and for them is a painful punishment because they [habitually] used to lie." Quran Surah Baqrah 2:10
Allaah Says (what means): "…Indeed Allaah does not guide one who is a transgressor and a liar." Quran Surah Baqrah Gafir 40:28
And (what means): "…Indeed, Allaah does not guide he who is a liar and [confirmed] disbeliever." Quran Surah Zumar 39:3
Almighty Allaah also Says (what means): "…The curse of Allaah be upon him if he should be among the liars." Quran Surah Nur 24:7
Allaah Almighty Says in the Quran (what means): "And if he [i.e., Muhammad] had made up about Us some [false] sayings, We would have seized by the right hand; then We would have cut from him the aorta " Quran Surah Baqrah Haqqah 69:44-46
Prophet Muhammadﷺ said: “It is obligatory for you to tell the truth, for truth leads to virtue and virtue leads to Paradise, and the man who continues to speak the truth and endeavours to tell the truth is eventually recorded as truthful with Allah, and beware of telling of a lie for telling of a lie leads to obscenity and obscenity leads to Hell-Fire, and the person who keeps telling lies and endeavours to tell a lie is recorded as a liar with Allah. " Sahih Muslim 2607
It was narrated that Abu Hurairah said: "The Messenger of Allah said: "There are three to whom Allah, it Mighty and Sublime, with not speak on the Day of Resurrection: An old man who commits adultery, a poor man who is arrogant, and an Imam who tells lies."' Sunan an-Nasa'i Vol.3: 2575
The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “It is enough lying for a man to speak of everything that he hears.” Sahih Muslim in al-Muqaddimah, 6 and Saheeh al-Jamee, 4482
Truthfulness is commanded by Allaah as a part of faith and is an indispensable quality of the believers. It is mentioned in over one hundred places in the Quran. Some examples are when Allaah Almighty Says (what means):
"That Allaah may reward the truthful for their truth…" Quran Surah 33:24
"O you who have believed! Fear Allaah and be with those who are true." Quran Surah 9:119
"O you who have believed! Fear Allaah and speak words of appropriate justice." Quran Surah 33:70
"The patient, the true, the obedient, those who spend [in the way of Allaah], and those who seek forgiveness before dawn." Quran Surah 3:17
"The believers are only the ones who have believed in Allaah and His Messenger and then doubt not but strive with their properties and their lives in the cause of Allaah. It is those who are the truthful." Quran Surah 49:15
It is not necessary to relate any prophetic traditions about the truthfulness of Prophet Muhammad ﷺ as his whole life was nothing but the epitome of truth. Prophet Muhammad ﷺ was confirmed as a truthful person even before he became a Prophet. During his prophethood, even his enemies confirmed that he was truthful and trustworthy. Some of the disbelievers would deposit their belongings with him as an indication of their trust in him.
Narrated Abu Umamah: The Prophet ﷺsaid: I guarantee a house in the surroundings of Paradise for a man who avoids quarrelling even if he were in the right, a house in the middle of Paradise for a man who avoids lying even if he were joking, and a house in the upper part of Paradise for a man who made his character good. Classed as Hasan by Shaik Albani in Sunan Abi Dawud 4800
Umm Kalthoom bint ‘Uqbah said: I never heard the Messenger of Allahﷺgrant a concession allowing any kind of lying except in three cases: a man who says something intending thereby to bring about reconciliation; a man who says something at the time of war; and a man talking to his wife or a woman talking to her husband. Imam Ahmad 26731, Classed as saheeh by al-Albaani in as-Saheehah, 545 The concession is granted only in these three cases and similar cases.