"Allah's Messenger (S) 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?'"
Reference: Jami` at-Tirmidhi 412 In-book : Book 2, Hadith 265 English translation : Vol. 1, Book 2, Hadith 412
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