It is narrated from Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said (to me): "Have I not been informed that you stand in prayer all night and fast all day?" I said: "Yes, I do that." He (peace and blessings be upon him) said: "If you do that, your eyes will sink in (and become weak). Fast three days every month; that is equal to fasting perpetually." I said: "I find in myself the strength (for more than that)." So he (peace and blessings be upon him) said: "Then observe the fast of Dawud (peace be upon him); he used to fast one day and break his fast one day."