Once a man came to Prophet Mohamed (pbuh) and said, "O Messenger of Allah, I have many bad habits. Which one of them should I give up first?" Prophet Mohamed (pbuh) 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 (pbuh) and his promise. "If tomorrow Prophet Mohamed (pbuh) 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 (pbuh) 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 (pbuh) 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 (SWT) likes and favours. If Allah (SWT) - 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 (pbuh) 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 (SWT) 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 (SWT) as a liar." (Muslim)
Imam Mohammad Al-Baqir (pbuh) says: "Allah (SWT) 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 (pbuh) 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 (pbuh) says that lying corrupts belief and Imam Ali Ibn Abi Talib (pbuh) says: "A person will not know the true meaning of faith until he abandons all types of lying."
Imam Ali Ibn Abi Talib (pbuh) 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."