Sahih Muslim Book 6, Hadith Number 2592.

Chapter : Forbiddance to observe perpetual fast and excellence of observing fast on alternate days.

Abdullah b. Amr (Allah be pleased with both of them) reported: The Messenger of Allah (may peace be upon him) said to me: ‘Abdullah b. Amr, you fast continuously and stand in prayer for the whole of night. If you do like that, your eyes would be highly strained and would sink and lose sight. There is no (reward for) fasting (for him) who fasts perpetually. Fasting for three days during the month is like fasting, the whole of the month. I said: I am capable of doing more than this, whereupon he said: Observe the fast of David. He used to fast one day and break (the other) day. And he did not turn back in the encounter.