Being truthful means speaking the truth and also saying things that reflect reality. Being truthful is one of the necessities of a human society, one of the virtues of human behavior, and brings great benefits, whilst lying is one of the major elements of corruption in human society, and the cause of the destruction of social structure and ties, one of the most evil features of bad conduct, and causes widespread harm. Hence Islam commanded truthfulness and forbade lying.
Allah says: “O you who believe! Be afraid of Allah, and be with those who are true (in word and deeds).” (9:119)
Ibn Katheer said: “It means: be truthful and adhere to truthfulness, and you will be among its people and will be saved from calamity, and this will make a way out for you from your problems.”
Allah says: “if they had been true to Allah, it would have been better for them.” (47:21)
Abd-Allah Ibn Mas’ood (radi allahu anhu – may Allah be pleased with him) said: The Messenger of Allah (sallallahu alaihi wa-sallam – may the peace & blessings of Allah be upon him) said: “You must be truthful, for truthfulness leads to righteousness and righteousness leads to Paradise. A man will keep speaking the truth and striving to speak the truth until he will be recorded with Allah as a siddeeq (speaker of the truth). Beware of telling lies, for lying leads to immorality and immorality leads to Hellfire. A man will keep telling lies and striving to tell lies until he is recorded with Allah as a liar.” (Muslim)
This Hadith indicates that truthfulness leads to righteousness, an all-embracing concept that includes all kinds of goodness and different kinds of righteous deeds. Immorality is basically an inclination towards deviation from the truth, and the immoral person is one who is inclined to turn away from the path of guidance. Hence immorality and righteousness are incompatible.
Al-Hasan Ibn Ali Ibn Abi Taalib (radi allahu anhu) said: I memorized from the Messenger of Allah (sallallahu alaihi wa-sallam): “Leave that which makes you doubt for that which does not make you doubt, for truthfulness is certainty and tranquility, whilst lying is doubt and confusion.” (Tirmidhi)
In the lengthy Hadith of Abu Sufyaan (radi allahu anhu) describing his meeting with Heraclius, Abu Sufyaan (radi allahu anhu) said: (Heraclius) said, “What does he (the Prophet sallallahu alaihi wa-sallam) command you to do?” I said, “He says: worship Allah alone and do not associate anything in worship with Him, and abandon that which your forefathers did. He commands us to pray, to be truthful, to be chaste and to uphold the ties of kinship.” (Bukhaari and Muslim)