Excelling in Acts of Worship

a'ishah, with whom is allah is pleased, said that the prophet of allah, may allah exalt his mention, used to pray during the night until his feet would swell.

a'ishah, may allah be pleased with her, said: 'why do you do this, o messenger of allah, and allah has forgiven your past and future sins?' the prophet, may allah exalt his mention, said: 'shall i not be a grateful slave?'  (bukhari #4557)