Musnad Ahmad 28:400

Muhammad ibn Ubayd narrated to us, Muhammad ibn Ishaq narrated to us, from Muhammad ibn Ibrahim, from Abu Salamah ibn Abd al-Rahman, who said: I entered upon Abdullah ibn Amr ibn al-Aas, and he asked me, thinking that I was the son of Umm Kulthum bint Uqbah. I said: "I am only the son of the Kalbiyyah woman." Abdullah said: The Messenger of Allah, peace and blessings of Allah be upon him, entered my house and said: "Have I not been informed that you recite the Quran every day and night? So recite it once every month." I said: "I am capable of more than that." He said: "Then recite it once every half-month." I said: "I am capable of more than that." He said: "Then recite it every seven days, and do not exceed that. And it has reached me that you fast perpetually?" I said: "I do indeed fast, O Messenger of Allah." He said: "Then fast three days of every month." I said: "I am capable of more than that." He said: "Then fast two days of every week." I said: "I am capable of more than that." He said: "Then fast the fast of David: fast one day and break your fast one day, for it is the most balanced of fasting before Allah. And he would not break his promise when he promised, and he would not flee when he encountered [the enemy]."