Sahih Bukhari Volume 004, Book 055, Hadith Number 629.
Narated By Abdullah bin Amr : Allah's Apostle was informed that I have said: "By Allah, I will fast all the days and pray all the nights as long as I live." On that, Allah's Apostle asked me. "Are you the one who says: 'I will fast all the days and pray all the nights as long as I live?' " I said, "Yes, I have said it." He said, "You cannot do that. So fast (sometimes) and do not fast (sometimes). Pray and sleep. Fast for three days a month, for the reward of a good deed is multiplied by ten time, and so the fasting of three days a month equals the fasting of a year." I said, "O Allah's Apostle! I can do (fast) more than this." He said, "Fast on every third day. I said: I can do (fast) more than that, He said: "Fast on alternate days and this was the fasting of David which is the most moderate sort of fasting." I said, "O Allah's Apostle! I can do (fast) more than that." He said, "There is nothing better than that."