Volume 4, Book 55, 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.”