Al-Lu'lu Wal-Marjan 718 — The Book of As-Siyâm (the Fasting)

'Abdullah bin 'Amr Narrated from:

The news of my observing Saum (fast) everyday and offering (voluntary) Salât (prayer) throughout the night reached the Prophet ﷺ. So he sent for me or I met him, and he said, “I have been informed that you observe Saum everyday and offer Salât every night (all the night). Observe Saum (for some days) and give up Saum (for some days); offer Salât and sleep, for your eyes have a right on you, and your body and your family (i.e. wife) have a right on you.”I replied, “I have more power than that (i.e. Saum).” The Prophet ﷺ said, “Then observe Saum like the Saum of (the Prophet) Dawûd (David).” I said, “How?” He replied, “He used to observe Saum on alternate days, and he used not to flee on meeting the enemy.” I said, “From where can I get that chance?” (‘Atâ’ said, “I do not know how the expression of Saum daily throughout the life occurred.”) So, the Prophet ﷺ said twice, “Whoever observes Saum daily throughout his life is just as the one who does not observe Saum at all.” ( Sahîh Al-Bukhârî, Hadîth No. 198, Vol. 3)