Musnad Ahmad 28:436

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Sa‘d ibn Ibrahim, from Hilal ibn Talhah or Talhah ibn Hilal, who said: I heard ‘Abdullah ibn ‘Amr saying: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: "O ‘Abdullah ibn ‘Amr, fast for the duration of time: three days of every month." He said: And he recited this verse: {Whoever comes with a good deed will have ten times the like thereof} [Al-An‘am: 160]. He said: I said: "I am capable of more than that." He said: "Fast the fast of David; he used to fast one day and break his fast one day."