Bukhari

Narrated `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).