Sahih Bukhari Volume 7, Book 72, Hadith Number 811.

Narrated By Abu Huraira : The Prophet said, “(Allah said), ‘Every good deed of Adam’s son is for him except fasting; it is for Me. and I shall reward (the fasting person) for it.’ Verily, the smell of the mouth of a fasting person is better to Allah than the smell of musk.”