(37) باب قَوْلِ اللَّهِ تَعَالَى {وَآتَيْنَا دَاوُدَ زَبُورًا}
Narrated `Abdullah bin `Amr bin Al-As:
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."
Reference: Sahih al-Bukhari 3419
In-book reference: Book 60, Hadith 91
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 4, Book 55, Hadith 630