Sahih Muslim Book 6, Hadith Number 2564.

Abu Huraira reported Allah’s Messenger (may peace be upon him) as saying: Allah, the Majestic and the Exalted, said: Every act of the son of Adam is for him except fasting. It is done for My sake, and I will give a reward for it. By Allah in Whose Hand is the life of Muhammad, the breath of the observer of fast is sweeter to Allah than the fragrance of musk.