(37) باب قَوْلِ اللَّهِ تَعَالَى {وَآتَيْنَا دَاوُدَ زَبُورًا}
Narrated `Abdullah bin `Amr:
Allah's Messenger (ﷺ) was informed that I have said: "By Allah, I will fast all the days and pray all the nights as long as I live." On that, Allah's Messenger (ﷺ) asked me. "Are you the one who says: 'I will fast all the days and pray all the nights as long as I live?' " I said, "Yes, I have said it." He said, "You cannot do that. So fast (sometimes) and do not fast (sometimes). Pray and sleep. Fast for three days a month, for the reward of a good deed is multiplied by ten time, and so the fasting of three days a month equals the fasting of a year." I said, "O Allah's Messenger (ﷺ)! I can do (fast) more than this." He said, "Fast on every third day. I said: I can do (fast) more than that, He said: "Fast on alternate days and this was the fasting of David which is the most moderate sort of fasting." I said, "O Allah's Messenger (ﷺ)! I can do (fast) more than that." He said, "There is nothing better than that."
Reference: Sahih al-Bukhari 3418
In-book reference: Book 60, Hadith 90
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 4, Book 55, Hadith 629