Truthfulness is a command of Allah (SUBHANA WA TAALA), part of faith, and a quality of must for all prophets and is mentioned in 100 places in the Noble Qur'an. Therefore, there are two positive traits that Allah (SUBHANA WA TAALA) wants the Muslim to acquire and be committed to in building his Islamic personality. Allah (SUBHANA WA TAALA) wants the believer to aspire among the people they live with a feeling of trust whether in what they say or in how they deal with them. They would feel that the money or other trusts they have with him are safe, while the things he says to them are true.
Being truthful and trustworthy is among the greatest virtues any human being can possess on the other hand lying leads to corruption and deceit in any society.
Prophet Mohamed (peace be upon him), as we are told by his sira (biography), was known in the forty years before he declare his Prophet Hood by his truthfulness and returning the trusts, to the extent that he was called: The truthful and the trustworthy. People use this phrase when they want to talk about him; the truthful and trustworthy said or did it, instead of saying Mohammed. Even his enemies confirmed that he was truthful as described by Abu Sufyan a leader of the tribe of Quraish and the arch enemy of Prophet Mohamed (peace be upon him) in the court of Hercules, the Byzantine emperor of Eastern Roman Empire.
Imam Ali Ibn al-Hussain (peace be upon him) said: "Refrain from lying in all things, big or small, in seriousness or in jest. For when one starts lying in petty matters, soon he will have the audacity to lie in important matters (as well)."
For example a joke like April fool's is prohibited just the same as serious lying. If man starts by small lies then he will end up making big lies, as the saying goes ... "He that will steal an egg will steal an ox."
The Noble Qur'an explicitly categorizes liars as disbelievers; "Only they forge the lies who do not believe in Allah's communications, and these are the liars." (16:105)
Imam Ali Ibn Abi Talib (peace be upon him) has said: "Lying is the most abominable trait."
Imam Hassan Al-Askari (peace be upon him) has said: "All spiteful traits are placed in a house and the key to this house is lying."
In order to further clarify what Imam Hassan Al-Askari (peace be upon him) said, we bring your attention to the following prophetic narration.
Once a man came to Prophet Mohamed (peace be upon him) and said, "O Messenger of Allah, I have many bad habits. Which one of them should I give up first?" Prophet Mohamed (peace be upon him) said, "Give up telling lies first and always speak the truth." The man promised to do so and never again commit another transgression and went home.
At night the man was about to go out to steal. Before setting out, he thought for a moment about the advice of Prophet Mohamed (peace be upon him) and his promise. "If tomorrow Prophet Mohamed (peace be upon him) asks me where have I been, what shall I say? Shall I say that I went out stealing? No, I cannot say that. But nor can I lie. If I tell the truth, everyone will start hating me and call me a thief. I would be punished for stealing."
So the man decided not to steal that night, and gave up this bad habit of stealing.
Next day, he felt like drinking wine, when he was about to do so, he said to himself, "What shall I say to Prophet Mohamed (peace be upon him) if he asks me what did I do during the day? I cannot tell a lie, and if I speak the truth people will hate me, because a Muslim is not allowed to drink wine." And so he gave up the idea of drinking wine.
In this way, whenever the man thought of doing something bad, he remembered the advice of Prophet Mohamed (peace be upon him) to tell the truth at all times. One by one, he gave up all his bad habits and became a good Muslim and a very good person.
If you always speak the truth, you can be a good person, a good Muslim whom Allah (SUBHANA WA TAALA) likes and favours. If Allah (SUBHANA WA TAALA) - our Creator - is pleased with us, HE will reward us with Heaven. This is a place of permanent joy and happiness.
Do you not know that Prophet Mohamed (peace be upon him) has 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 (SUBHANA WA TAALA) 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 (SUBHANA WA TAALA) as a liar." (Muslim)
Imam Mohammad Al-Baqir (peace be upon him) says: "Allah (SUBHANA WA TAALA) has made locks for evil and made drinking the keys to these locks." Since drinking makes one loses his mind and ultimately commits evil. Imam Mohammad Al-Baqir (peace be upon him) added: "But lying is more evil than drinking." Drinking makes man loses his mind and unintentionally hurt others, but the liar commits evil deeds intentionally. That is why Imam Mohammad Al-Baqir (peace be upon him) says that lying corrupts belief and Imam Ali Ibn Abi Talib (peace be upon him) says: "A person will not know the true meaning of faith until he abandons all types of lying."
Imam Ali Ibn Abi Talib (peace be upon him) stated the clear benefits of truthfulness: "The truth teller achieves three things: (other's) trust, love, and respect." And warns at another place by saying: "Be not mislead by their prayers and fasting, for a man can become fervently used to prayers and fasting so that if he would abandon them he would feel lonely. Rather, try them when it comes to telling the truth and fulfilling trusts."