On the authority of Abu Harayrah (may Allah be pleased with him) from the Prophet (PBUH), who said: Allah (mighty and sublime be He) says:

Fasting is Mine and it I who give reward for it. [A man] gives up his sexual passion, his food and his drink for my sake. Fasting is like a shield, and he who fasts has two joys: a joy whin he breaks his fast and a joy when he meets his Lord. The change in the breath of the mouth of him who fasts is better in Allah's estimation than the smell of musk.

It was related by al-Bukhari (also by Muslim, Malik, at-Tirmidhi, an-Nasa'i and Ibn Majah).