A man came to Prophet Muhammad (pbuh) and said that he had some bad habits and asked Muhammad (pbuh) for some advice. The prophet Muhammad (pbuh) advised him to always speak the truth.

The next day the man was going to steal something. He thought what will I tell the prophet (pbuh) if he asks me what I did today. He knew that he would have to tell the prophet (pbuh) the truth and so decided not to steal.

The next day the same man wanted to drink alcohol. He again thought what will I tell the prophet (pbuh) if he asks me, "What did you do today?" He decided to leave this also as he knew he had to tell the truth to the Prophet (pbuh).

From that day on the man always tried his best to tell the truth and all his bad habits disappeared.

Abdullah ibn Masood(r.a.a) relates that the Prophet
(s.a.w) said 'A true action leads to the path of
virtue and good deeds, and virtue paves the way of a
person to Paradise, and the said person continues to
speak the truth till in the sight of Allah he is named
Siddiq (Truthful). lying leads to vice, and vice
leads to indecent acts and a person goes on lying till
in the sight of Allah he is named a liar.'

(Bukhari and Muslim)
from Riyadh us-Saleheen, hadith #54