Musnad Abu Ya'la al-Mawsili 54:4

Ahmad ibn Isa narrated to us, Ibn Wahb narrated to us, Yahya ibn Ayyub informed me, from Zabban ibn Fa'id, from Sahl ibn Mu'adh, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever fasts one day in the way of Allah, voluntarily, outside of Ramadan, Allah will distance him from the Fire by one hundred years of the journey of a swift, well-trained racehorse."