Musnad Ahmad 30:872

Anas ibn Iyad narrated to us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever fasts one day in the way of Allah, Allah will distance his face from the Fire by that [act] for seventy autumns."