Musnad Ahmad 28:284

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Ibn al-Musayyib and Abu Salamah ibn 'Abd al-Rahman, from 'Abd Allah ibn 'Amr ibn al-'As, who said: The Messenger of Allah (peace and blessings of Allah be upon him) met me and said: "Have I not been informed that you stand [in prayer] at night? Or: Are you the one who says: 'I will surely stand [in prayer] at night and I will surely fast during the day'?" He said: I think he said: Yes, O Messenger of Allah, I have said that. He said: "So stand [in prayer] and sleep, and fast and break your fast, and fast three days of every month, for you have the like of fasting the [entire] year." I said: O Messenger of Allah, I am capable of more than that. He said: "Then fast one day and break your fast for two days." I said: I am capable of better than that. He said: "Then fast one day and break your fast one day, and it is the most balanced of fasting, and it is the fasting of David." I said: I am capable of better than that. The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is nothing better than that."