Al-Lu'lu Wal-Marjan 715 — The Book of As-Siyâm (the Fasting)

Narrated 'Abdullâh bin 'Amr bin Al-'Âs رضي الله عنه : Allâh's Messenger ﷺ said to me, "O 'Abdullâh! Have I not been informed that you observe Saum (fast) during the day and offer Salât (prayer) all the night." 'Abdullâh replied, "Yes, O Allâh's Messenger!" The Prophet ﷺ said, "Don't do that; observe Saum for few days and then give it up for few days, offer Salât 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. It is sufficient for you to observe Saum three days in a month; as the reward of good deeds is multiplied ten times, so it will be like observing Saum throughout the year." I insisted (on observing Saum ) and so I was given a hard instruction. I said, "O Allâh's Messenger! I have strength." The Prophet ﷺ said, "Observe Saum like the Saum of the Prophet Dawûd (David) ﷺ and do not observe Saum more than that." I asked, "How was the Saum of the Prophet of Allâh, Dawûd ﷺ?" He said, "Half of the year (i.e. he used to observe Saum on every alternate day)." Afterwards when 'Abdullâh 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 observe Saum only three days a month)." ( Sahîh Al-Bukhârî, Hadîth No. 196, Vol. 3 )