Musnad Ahmad 30:2199

Muhammad ibn Ja‘far narrated to us, Hisham ibn Hassan al-Qardusi narrated to us, from Muhammad ibn Sirin, from Abu Hurayrah, from the Prophet, may Allah bless him and grant him peace, who said: "A good deed is [rewarded] by ten times its like. And fasting is for Me, and I reward for it. He leaves his food and his drink for My sake. Fasting is for Me, and I reward for it." "And the change in the breath of the mouth of the fasting person is, in the sight of Allah, more pleasant than the scent of musk."