Musnad Ahmad 28:619

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Ziyad ibn Fayyad, I heard Abu 'Iyad narrating from Abdullah ibn 'Amr, that the Prophet, peace and blessings of Allah be upon him, said to him: "Fast one day, and you will have the reward of what remains." He said: "I am capable of more than that." He said: "Fast two days, and you will have the reward of what remains." He said: "I am capable of more than that." He said: "Fast three days, and you will have the reward of what remains." He said: "I am capable of more than that." He said: "Fast four days, and you will have the reward of what remains." He said: "I am capable of more than that." He said: "Fast the best of fasting in the sight of Allah; fast the fasting of David, for he used to fast one day and break his fast one day."