Musnad Ahmad 937:62

Muhammad ibn Ubayd narrated to us, al-A’mash narrated to us, from Yahya ibn Sam, from Musa ibn Talhah, from Abu Dharr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever among you is fasting three days of the month, let him fast the three white days (al-ayyam al-bid)."