Musnad Ahmad 31:416

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Suhayl ibn Abi Salih, from Safwan, from Abu Sa'id al-Khudri, from the Prophet, peace and blessings of Allah be upon him, that he said: "Whoever fasts one day in the way of Allah, Allah will distance his face from Hell a distance of seventy years."