Musnad Ahmad 30:3423

Yazid narrated to us, Shu'bah informed us, from Muhammad ibn Ziyad, from Abu Hurayrah, from the Prophet, may God bless him and grant him peace, narrating it from his Lord, the Almighty and Majestic, who said: "For every deed there is an expiation, and fasting is for Me, and I reward for it." "And the change in the breath of the mouth of the fasting person is more pleasant to God than the scent of musk."