Musnad Ahmad 30:670

'Abd al-Razzaq narrated to us, Ma'mar and 'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from Ibn al-Musayyib, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Allah, the Mighty and Majestic, said: 'Every deed of the son of Adam is for him, except for fasting; fasting is for Me, and I shall reward for it.' And the change in the breath of the mouth of the fasting person is more pleasant to Allah than the scent of musk."