'Anas-ibn-Mlaik narrates from the Messenger of Allah (S) who said: "Accept six things from me, then I will accept (and guarantee) the entrance of Heaven for you:
1. When you speak, tell no lies.
2. When you promise, do not break it.
3. When you are trusted (by others), be honest.
4. Lower your eyes (from committing sin).
5. Guard your modesty.
6. Hold your hands and tongues."
Khisal al-Saduq, p. 321
Imam Abi-Muhammad-al-al-’Askari, the eleventh Imam, (‘a) said: "Wickedness was put inside a house, and the key (of its door) was rendered to be lies."
Bihar-ul-Anwar, vol. 72, p. 263
Imam Sajjad (‘a) said: "Avoid telling lies irrespective of small or great, whether it is in earnest or joking, because when a person tells a lie in small (things), he dares to tell a lie in great things, too."
Tuhaful-Uqul, p. 201
Once Imam Amir-ul-Mu'mineen ‘Ali (‘a) was asked about the distance between the truth and untruth and, while he (‘a) was showing the distance between his eye and ear with his four fingers in order to illustrate this distance as his answer, he said: "What your eyes see is 'truth' and what your ears hear is for the most part 'untruth'."
Bihar-ul-Anwar, vol. 75, p. 196
Imam al-Baqir (‘a) said: "Verily, Allah, the exalted, has put some locks on vices, the key of which He has put in wine, but the vice of lying is worse than wine."
Al-Kafi, vol. 2, p. 339
Imam Ridha’ (‘a) said: "Be truthful and avoid telling lies."
Bihar-ul-Anwar, vol. 78, p. 347