Sahih Bukhari Volume 003, Book 031, Hadith Number 200.
Narated By 'Abdullah bin 'Amr bin Al-'As : The Prophet said to me, "You fast daily all the year and pray every night all the night?" I replied in the affirmative. The Prophet said, "If you keep on doing this, your eyes will become weak and your body will get tired. He who fasts all the year is as he who did not fast at all. The fasting of three days (a month) will be equal to the tasting of the whole year." I replied, "I have the power for more than this." The Prophet said, "Then fast like the fasting of David who used to fast on alternate days and would never flee from the battle field, on meeting the enemy.