Musnad Ahmad 30:1569

Ishaq narrated to us, Abd al-Rahman ibn Zayd narrated to us, from his father, from Abu Salih al-Samman, from Abu Hurayrah, who said: 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 him from Hell a distance of seventy autumns."