Allah's Messenger (ﷺ) 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 Messenger (ﷺ)!" The Prophet (ﷺ) 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 Messenger (ﷺ)! I have power." The Prophet (ﷺ) said, "Fast like the fasting of the Prophet (ﷺ) David
and do not fast more than that." I said, "How was the fasting of the Prophet (ﷺ) 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).
Sahih al-Bukhari Book 30, Hadith 82