narrated 'abdullah bin 'amr bin al-'as (radi-allahu 'anhu):
allah's apostle (sallallahu 'alaihi wa sallam) said, "o 'abdullah! have i not been formed that you fast all the day and stand in prayer all night?" i said, "yes, o allah's apostle!" he said, "do not do that! observe the fast sometimes and also leave them (the fast) at other times; stand up for the prayer at night and also sleep at night. your body has a right over you, your eyes have a right over you and your wife has a right over you."
bukhari vol. 7 : no. 127