Sahih Bukhari Volume 003, Book 031, Hadith Number 198.
Narated By 'Abdullah bin 'Amr : The news of my daily fasting and praying every night throughout the night reached the Prophet. So he sent for me or I met him, and he said, "I have been informed that you fast everyday and pray every night (all the night). Fast (for some days) and give up fasting (for some days); pray and sleep, for your eyes have a right on you, and your body and your family (i.e. wife) have a right on you." I replied, "I have more power than that (fasting)." The Prophet said, "Then fast like the fasts of (the Prophet) David". I said, "How?" He replied, "He used to fast on alternate days, and he used not to flee on meeting the enemy." I said, "From where can I get that chance?" ('Ata' said, "I do not know how the expression of fasting daily throughout the life occurred.") So, the Prophet said, twice, "Whoever fasts daily throughout his life is just as the one who does not fast at all."