The Prophets' example: Rasulullah (saws) was known to be a very truthful and trustworthy individual. Even before Allah (s.w.t) appointed him to be the last and final prophet and messenger he was known amongst his people for his exceptionally honest character. They even called him Al-Amin which means the trustworthy and as-Sadiq which means the truthful one. Rasulullah (saws) is reported to have said:
If anyone is pleased to love Allah (s.w.t) and His messenger or rather to have Allah (s.w.t) and His messenger love him, he should speak the truth when he says anything, and fulfill his trust when he is in a position of trust (Transmitted by Baihaqi).
What the Quran says: The Quran teaches us to be truthful and trustworthy in many places. For example:
And cover not the Truth with falsehood, nor conceal the Truth when ye know (what it is). (2:42).
O you who believe! Fulfill all obligations. (5:1).
O you who believe! Why say you that which you do not? Grievously hateful is it in the sight of Allah that you say that which you do not. (61:2-3).
This is why we as Muslims must put our utmost effort to avoid telling lies. One lie often leads to others. As Muslims we must also make sure that whenever we make a promise we keep it to the best of our ability. Only then will we gain the respect and trust of the people and Inshallah we will gain the pleasure of Allah (s.w.t).