Musnad Ahmad 31:369

Mu'awiyah narrated to us, Shayban narrated to us, from Firas, from 'Atiyyah, from Abu Sa'id, from the Prophet of Allah, peace be upon him, that he said: "The change in the breath of the fasting person is more pleasant to Allah than the scent of musk. He says: 'He fasted for My sake, and he left his desire for food and drink for My sake. So the fasting is for Me, and I shall reward for it.'"