Sahih Muslim 2819 a
Mughira b. Shu'ba reported that Allah's Apostle (ﷺ) 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)?
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ زِيَادِ بْنِ عِلاَقَةَ، عَنِ الْمُغِيرَةِ بْنِ،
 شُعْبَةَ أَنَّ النَّبِيَّ صلى الله عليه وسلم صَلَّى حَتَّى انْتَفَخَتْ قَدَمَاهُ فَقِيلَ لَهُ أَتَكَلَّفُ هَذَا وَقَدْ
 غَفَرَ اللَّهُ لَكَ مَا تَقَدَّمَ مِنْ ذَنْبِكَ وَمَا تَأَخَّرَ فَقَالَ ‏
"‏ أَفَلاَ أَكُونُ عَبْدًا شَكُورًا ‏"‏ ‏.‏