Sahi Bukhari Volume 4, Number : 630

Sahi Bukhari
Volume 4, Number : 630

Narrated `Abdullah bin `Amr bin Al−As: The Prophet 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."