Musnad Ahmad 31:570

Ibn Numayr narrated to us, Sufyan ibn Sa'id ibn Masruq informed us, from Sumayy, from al-Nu'man ibn Abi 'Ayyash, from Abu Sa'id al-Khudri, 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 the Fire from his face by that day [a distance of] seventy autumns."