Musnad Ahmad 30:2895

'Abd al-Rahman ibn Mahdi narrated to us, from Hammad ibn Salamah, from Muhammad ibn Ziyad, who said: I heard Abu Hurayrah saying: I heard Abu al-Qasim (may Allah bless him and grant him peace) saying: "Allah the Almighty and Majestic said: 'Every deed is an expiation, except for fasting; for fasting is for Me, and I shall reward for it.'"