Sahih Muslim Book 39, Hadith Number 6772.

Chapter : The doing of good deeds very frequently and putting one’s best efforts in devotion to Allah.

Mughira b. Shu’ba reported that Allah’s Apostle (may peace be upon him) worshipped so much that his feet were swollen. It was said to him: (Why do you undergo so much hardship despite the fact that) Allah has pardoned for you your earlier and later sins? Thereupon he said: May I not (prove myself) to be a grateful servant (of Allah)?