Maliks Muwatta Book 18, Hadith Number 58.
Section : The Fast in General.
Yahya related to me from Malik from Abu’z Zinad from al-Araj from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, “By the One in Whose hand my self is, the smell of the breath of a man fasting is better with Allah than the scent of musk.’ He leaves his desires and his food and drink for My sake. Fasting is for Me and I reward it. Every good action is rewarded by ten times its kind, up to seven hundred times, except fasting, which is for Me, and I reward it.'”