The affairs of the world proceed upon implied trust and truthfulness in reports and dealings, and people are inclined towards truthfulness and honesty from their innate disposition (fiṭrah). Had this not been the case, then all of the affairs of the world would be corrupted. Hence, lying is from the most repugnant of sins, and is the foundation of evils, just as truthfulness is the foundation of goodness.
This is why due to its repugnance, it is not conceivable that a person of Tawḥid and Sunnah should lie.
Shaykh Rabīʿ bin Hādī said:[1]
By Allāh, I cannot imagine that a Salafī would lie.
As for the definition of lying, then Ḥāfidh Ibn Ḥajar said:[2]
Lying is to inform about something different to what it is, whether intentionally or by mistake.
Ḥāfidh al-Nawawī said:[3]
Know that the position of Ahl al-Sunnah is that lying is to inform about something different to what it is, whether you did it knowingly or unknowingly (in ignorance). However, you are not sinful if you did it in ignorance. Sin is when it is doing knowingly.
Al-Rāghib al-Aṣfahānī said:[4]
A lie is either an invention (ikhtirāʾ) of a story that has no basis, an addition or subtraction (of detail) to a story that changes the meaning, or a distortion (of it) by changing a phrase. Whatever is an invention is called a fabrication (iftirāʾ, ikhtilāq), and whatever is an addition or subtraction (to the report, story) is called a lie (kadhib). Everyone who narrated a lie (kadhib) about someone else, either says it in his presence or without his presence. The greatest lie is that which is invented in the presence of the one being spoken about, and this is what is explained as slander (buhtān).[5]
And al-Rāghib also said:[6]
Lying is a permanent disgrace and a permanent humiliation. It is the duty of a man to seek the truth, be accustomed to it and not entertain even the slightest lie. Whoever permits it (for himself) wil find it difficult to wean (himself) from it. One of the wise men said: ‘Every sin can be repented from and shown remorse except lying, for its perpetrator becomes more arrogant. We have seen a drunkard leave (his sin), a thief withhold (from stealing), but we have never seen a liar return (from his lies).
Shaykh ʿAbd al-Raḥmān al-Muʿallimī:[7]
The majority of the people of knowledge hold that if a report agrees with the reality, then it is truthfulness, (ṣīdq) and if not, then a lie (kadhib). This is the saying of Ahl al-Sunnah as occurs in al-Miṣbāḥ (al-Munīr) and its evidences are numerous. However, in customary usage, it is not used for the one who lied in error, (such that it is said): “So-and-so-lied.” Rather, it is said: “So-and-so erred” (akhṭāʾa fulān, ghaliṭā fulān), unless the intent is to attribute negligence to him, as a deterrent to him from doing such a thing, or the intent is to confirm the ruling of the invalidity of the report, or the intent is to denounce the informer to discourage people from following him. It is upon these three examples, which Ibn ʿAbd al-Barr mentioned in his book “al-ʿIlm” that the word lying (kadhib) can be applied to error (khaṭāʾ).
He also said:[8]
Blame and sin are not attributed to everyone who reports news contrary to the reality. Rather, the one who errs is excused if he is not negligent, but (blame and sin) are attributed to the one who is reckless (in reporting). The one who reports (a lie) can also be attributed the disgrace and sin of lying, even if the report itself is true. This is when he asserts (the veracity) of the report, even though he sees it as a lie, being untrue, or he does not know that it is true. In such a case, blame and sin are attributed to him.
Summary
A lie is to inform about something upon other than its actual reality. This is done either unknowingly, which still technically makes it a lie, and it can be referred to a lie (kadhib) and the person to have lied in saying it in particular situations: a) as a deterrent from negligence, or b) to prevent people from following the one who said it, or c) to confirm the ruling of invalidity for it.
And if it is done knowingly, then if it is a pure invention, it is a fabrication (iftirāʾ), and if it is altering details (such as by addition, subtraction, omission and the likes) so that it does not conform with reality, it is a lie (kadhib). And that which is fabricated in someone’s presence is slander (buhtān) and that which is fabricated and carried without verification, that is slander (ifk).
Lying is a disgrace and the one who falls into it will find it very difficult to wean himself from it, it will lead him to arrogance (because he has to maintain the lie to protect himself). Further, the lie that emanates from the heart, by necessity spreads to the tongue and then the limbs, and eventually, lying encompasses all of a person, until everything becomes a lie, and refuge is with Allāh.