Sayyiduna Abdullah bin Amr bin Al-As (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings be upon him) said to me: "Do you fast continuously and stand in prayer all night?" I replied in the affirmative. He said: "If you continue doing so, your eyes will sink in and you will become extremely weak. This is not fasting, that a person fasts every day of his life (without a break). Fasting three days (in every month) is equal to fasting for a lifetime." I said upon this, "I have the strength for more than that." He said: "Then observe the fast of Dawud (peace be upon him); he used to fast one day and leave fasting the next day, and when he faced the enemy, he would not turn his back."