Sahih Bukhari Volume 9, Book 93, Hadith Number 629.

Narrated By Abu Huraira : The Prophet said that your Lord said, “Every (sinful) deed can be expiated; and the fast is for Me, so I will give the reward for it; and the smell which comes out of the mouth of a fasting person, is better in Allah’s Sight than the smell of musk.” (See Hadith No. 584)