Musnad Ahmad 937:228

Sulayman ibn Dawud al-Hashimi narrated to us, Isma'il—meaning Ibn Ja'far—informed us, Muhammad ibn Abi Harmalah informed me, from 'Ata' ibn Yasar, from Abu Dharr, who said: My beloved advised me with three things that I shall never abandon, if Allah wills: He advised me to perform the Duha prayer, to perform the Witr prayer before sleeping, and to fast three days of every month.