Musnad Ahmad 28:367

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Amr ibn Dinar, from Abu al-‘Abbas, who narrated from ‘Abdullah ibn ‘Amr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: "Recite the Quran in a month." I said: "I am capable of more than that." I continued to request him until he said: "Recite the Quran in five days, and fast three days of the month." I said: "I am capable of more than that." He said: "Then fast the most beloved fast to Allah the Almighty, the fast of Dawud (peace be upon him); he used to fast one day and break his fast one day."