Abu Hurairah (May Allah be pleased with him) reported:
Messenger of Allah (PBUH) said,"Three are the signs of a hypocrite:
1) When he speaks, he lies;
2) When he makes a promise, he breaks it;
3) And when he is trusted, he betrays his trust.''
[Al-Bukhari and Muslim]
Another narration adds the words: "Even if he observes Saum (fasts), performs Salat (prayer) and claims to be a Muslim.''
Riyad-us-Saliheen – Hadith No : 689