The Prophet (ﷺ) said to me, "I have been informed that you pray all the nights and observe fast all the days; is this true?" I replied, "Yes." He said, "If you do so, your eyes will become weak and you will get bored. So fast three days a month, for this will be the fasting of a whole year, or equal to the fasting of a whole year." I said, "I find myself able to fast more." He said, "Then fast like the fasting of (the Prophet) David who used to fast on alternate days and would not flee on facing the enemy."
Sahih al-Bukhari Book 60, Hadith 91
← Previous Hadith 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."

Sahih al-Bukhari Book 60, Hadith 90 Next Hadith → Allah's Messenger (ﷺ) said to me, "The most beloved fasting to Allah was the fasting of (the Prophet) David who used to fast on alternate days. And the most beloved prayer to Allah was the prayer of David who used to sleep for (the first) half of the night and pray for 1/3 of it and (again) sleep for a sixth of it."

Sahih al-Bukhari Book 60, Hadith 92