Musnad Ahmad 28:391

Muhammad ibn Mus'ab narrated to us, al-Awza'i narrated to us, from Yahya, from Abu Salamah ibn 'Abd al-Rahman, from 'Abdullah ibn 'Amr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: "I have been informed that you stand [in prayer] at night and fast during the day?" I said: "Yes, O Messenger of Allah." He said: "Fast and break your fast, pray and sleep, for your body has a right over you, your wife has a right over you, and your guest has a right over you. It is sufficient for you to fast three days every month." I insisted, so he made it harder for me. I said: "O Messenger of Allah, I find strength." He said: "Then fast three days every week." I insisted, so he made it harder for me. I said: "O Messenger of Allah, I find strength." He said: "Fast the fast of the Prophet of Allah, David, and do not exceed it." I said: "O Messenger of Allah, and what was the fast of David?" He said: "He used to fast one day and break his fast one day."