Sahih Muslim 2819 a

The Prophet (ﷺ) used to stand (in the prayer) or pray till both his feet or legs swelled. He was asked why (he offered such an unbearable prayer) and he said, "should I not be a thankful slave."

The Prophet (ﷺ) 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)?"

The Prophet (ﷺ) used to offer night prayers till his feet became swollen. Somebody said, to him," "Allah has forgiven you, your faults of the past and those to follow." On that, he said, "Shouldn't I be a thankful slave of Allah)?"

The Prophet (ﷺ) used to offer prayer at night (for such a long time) that his feet used to crack. I said, "O Allah's Messenger (ﷺ) ! Why do you do it since Allah has forgiven you your faults of the past and those to follow?" He said, "Shouldn't I love to be a thankful slave (of Allah)?' When he became old, he prayed while sitting, but if he wanted to perform a bowing, he would get up, recite (some other verses) and then perform the bowing.

Allah's Messenger (ﷺ) kept standing in prayer (for such long hours) that his feet were swollen. They (his Companions) said: Verily, Allah has pardoned for you the earlier and the later of your sins. Thereupon he said: Should I not prove myself to be a grateful servant (of Allah)?

Allah's Messenger (ﷺ) you do this (in spite of the fact) that your earlier and later sins have been pardoned for you? Thereupon, he said. A'isha should I not prove myself to be a thanksgiving servant (of Allah)?