Abu Hurairah (May Allah be pleased with him) reported: Messenger of Allah (PBUH) said, "Three are the signs of a hypocrite: When he speaks, he lies; when he makes a promise, he breaks it; 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.''