Sahih al-Bukhari 3419
صحيح البخاري#3419SahihNarrated `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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 60:91