Sahih Bukhari Volume 006, Book 061, Hadith Number 572.
Narrated By 'Abdullah bin Amr bin Al-As : My father got me married to a lady of a noble family, and often used to ask my wife about me, and she used to reply, "What a wonderful man he is! He never comes to my bed, nor has he approached me since he married me." When this state continued for a long period, my father told the story to the Prophet who said to my father, "Let me meet him." Then I met him and he asked me, "How do you fast?" I replied, "I fast daily," He asked, "How long does it take you to finish the recitation of the whole Quran?" I replied, "I finish it every night." On that he said, "Fast for three days every month and recite the Qur'an (and finish it) in one month." I said, "But I have power to do more than that." He said, "Then fast for three days per week." I said, "I have the power to do more than that." He said, "Therefore, fast the most superior type of fasting, (that is, the fasting of (prophet) David who used to fast every alternate day; and finish the recitation of the whole Qur'an In seven days." I wish I had accepted the permission of Allah's Apostle as I have become a weak old man. It is said that 'Abdullah used to recite one-seventh of the Qur'an during the day-time to some of his family members, for he used to check his memorization of what he would recite at night during the daytime so that it would be easier for him to read at night. And whenever he wanted to gain some strength, he used to give up fasting for some days and count those days to fast for a similar period, for he disliked to leave those things which he used to do during the lifetime of the Prophet.