Musnad Ahmad 937:148

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Sulayman, from Yahya ibn Sam, from Musa ibn Talhah, from Abu Dharr, that he said: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: "If you fast three days of the month, then fast the thirteenth, the fourteenth, and the fifteenth."