Musnad Ahmad 28:437

Rawh narrated to us, Shu'bah narrated to us, from Ziyad ibn Fayyad, from Abu 'Iyad, I heard Abdullah ibn 'Amr saying: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: "Fast one day, and you will have the reward of what remains," until he counted four days or five—Shu'bah is uncertain. He said: "Fast the best of fasting, the fasting of Dawud (peace be upon him); he used to fast one day and break his fast one day."