Narrated Al-Mughira bin Shu'ba (Radi-Allahu 'anhu):
The Prophet (Sallallahu 'Alaihi Wa Sallam) used to pray so much that his
feet used to become oedematous or swollen, and when he was asked as to why
he prays so much, he would say, "Shall I not be a thankful slave (to Allah)?"
Bukhari Vol. 8 : No. 478