Telling the truth is a very good habit. If you always speak the truth, you can save yourself from a lot of trouble! Here is a story of a man who did a lot of bad things, but his promise to tell the truth saved him.
Once a man came to the prophet Muhammad (PBUH) and said, "Oh prophet of Almighty Allah, I have many bad habits. Which one of them should I give up first?" The prophet said, "Give up telling lies first and always speak the truth." The man promised to do so and went home.
At night the man was about to go out to steal. Before setting out, he thought for a moment about the promise he made with the prophet. "If tomorrow the prophet 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.
Next day, he felt like drinking wine, when he was about to do so, he said to himself, "What shall I say to the prophet 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 his promise 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 likes and favors. If Allah - our Creator - is pleased with us, He will reward us with HEAVEN, which is a place of happiness and joy.
MAKE A PROMISE: I SHALL ALWAYS SPEAK THE TRUTH.
O ye who believe! stand out firmly for justice, as witnesses to Allah, even as against yourselves, or your parents, or your kin, and whether it be (against) rich or poor: for Allah can best protect both. Follow not the lusts (of your hearts), lest ye swerve, and if ye distort (justice) or decline to do justice, verily Allah is well-acquainted with all that ye do. Holy Quran - 4:135