Musnad Ahmad 494:1

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Anas ibn Sirin, from 'Abd al-Malik ibn al-Minhal, from his father, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, commanded us [to fast] the white days, for it is the fasting of the month."