Musnad Ahmad 26:310

Abu al-Nadr and Hasan narrated to us, they said: Shayban narrated to us, from 'Asim, from Zirr, from 'Abdullah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, used to fast three days from the beginning of every crescent moon, and he rarely used to break his fast on a Friday.