Musnad Ahmad 30:2760

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, from Muhammad ibn Ziyad, from Abu Hurairah, from the Prophet, may Allah bless him and grant him peace, narrating it from your Lord, the Almighty and Majestic: "Every deed 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 Allah than the scent of musk."