Sahih Muslim Book 6, Hadith Number 2589.

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

‘Abdullah b. ‘Amr (Allah be pleased with them) reported: The Messenger of Allah (may peace be upon him) said to me: Recite the whole of the Qur’an during every month. I said: I find power (to recite it) in a shorter period. He said: Then recite it in twenty nights. I said: I find power (to recite it in a shorter period even than this), whereupon he said: Then recite it in seven (nights) and do not exceed beyond it.