Al-Mughirah bin Shu’bah narrated: “Allah’s Messenger (peace be upon him) performed Salat until his feet were swollen, so it was said to him: ‘You burden yourself like this, while your past and future sins have been forgiven?’ He said: ‘Shouldn’t I be a grateful worshipper?'”
Jami` at-Tirmidhi, Vol, 1, Book 2, Hadith 265
