Musnad Ahmad 937:46

Sufyan narrated to us, he said: We heard it from two or three [people], Hakim ibn Jubayr narrated to us, from Musa ibn Talhah, from Ibn al-Hutakiyyah, he said: Umar said, "Who among us was present on the day of al-Qahah?" Abu Dharr said, "I was." The Messenger of Allah, peace and blessings of Allah be upon him, commanded him to fast the bright white days: the thirteenth, the fourteenth, and the fifteenth.