Silsilat al-Ahadith al-Sahihah

Sayyiduna Abdullah bin Amr (may Allah be pleased with him) says: The Messenger of Allah (peace and blessings be upon him) said to me: "O Abdullah bin Amr! You fast all the time and stand in prayer all night. If you continue with these deeds, your eyes will sink and you will become weak and frail. (Remember that) the person who fasts continuously has not really fasted. Do it like this: fast three days every month; this is equal to fasting the whole month." I said: I have strength for more than this. The Prophet (peace and blessings be upon him) said: "Then observe the fast of Dawud (peace be upon him); he would fast one day and break the fast the next day, and when (he faced the enemy) he would not flee."