Bismillah irrahman irrahim
Narrated Al-Mughira bin Shu'ba radi allaho anho: The Prophet Sallallaho Alaihey Wasallam used to pray so much that his feet used to become edematous 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)?"
Sahih Bukhari, Volume 8, Book 76, Number 478:

Mafhum-e-Hadith:
Hazard Al-Mughira bin Shu'ba Radiallaho anho se rivayat hai ki Nabee Sallallaho Alaihey Wasallam itni namaz padte they ki Aapke qadmo mein waram (swelling) aa jata ya aapke Qadam Full jatey jab Aap Sallallaho Alaihey Wasallam se arz kiya jata ki Aap itni namaz kyun padte ho to Aap Sallallaho Alaihey Wasallam ne farmaya ki Kya mein Allah ka ShukarGuzar Banda na banu
Sahih bukhari, Dil ko naram karney wali baatein (6471)