Musnad Ahmad 31:789

Yazid narrated to us, Hammad ibn Salamah informed us, from Suhayl ibn Abi Salih, from al-Nu’man ibn Abi ‘Ayyash, from Abu Sa’id al-Khudri, from the Prophet, peace and blessings of Allah be upon him, who said: "Whoever fasts one day in the way of Allah, Allah will distance between him and the Fire a distance of seventy autumns."