Sahih Muslim Book 6, Hadith Number 2599.

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

‘Abdullah b. ‘Amr (Allah be pleased with them) reported that the Messenger of Allah (may peace be upon him) said to me ! ‘Abdullah b. ‘Amr, it has been conveyed to me that you observe fast during the day and stand in prayer during the whole night. Don’t do that, for your body has a share of its own in you, your eye has a share of its own in you, your wife has a share of her own in you. Observe fast and break it too. Fast for three days in every month and that is a perpetual fasting. I said! Messenger of Allah, I have got strength enough (to do more than this), whereupon he said: Then observe the fast of David (peace be upon him). Observe fast one day and break it (on the other) day. And he (‘Abdullah b. ‘Amr) used to say: Would that I had availed myself of this concession.