Sahih Bukhari Volume 3, Book 31, Hadith Number 196.
Narated By ‘Abdullah bin ‘Amr bin Al-‘As : Allah’s Apostle said to me, “O ‘Abdullah! Have I not been informed that you fast during the day and offer prayers all the night.” ‘Abdullah replied, “Yes, O Allah’s Apostle!” The Prophet said, “Don’t do that; fast for few days and then give it up for few days, offer prayers and also sleep at night, as your body has a right on you, and your wife has a right on you, and your guest has a right on you. And it is sufficient for you to fast three days in a month, as the reward of a good deed is multiplied ten times, so it will be like fasting throughout the year.” I insisted (on fasting) and so I was given a hard instruction. I said, “O Allah’s Apostle! I have power.” The Prophet said, “Fast like the fasting of the Prophet David and do not fast more than that.” I said, “How was the fasting of the Prophet of Allah, David?” He said, “Half of the year,” (i.e. he used to fast on every alternate day).
Afterwards when ‘Abdullah became old, he used to say, “It would have been better for me if I had accepted the permission of the Prophet (which he gave me i.e. to fast only three days a month).”