كتاب صفة القيامة والجنة والنار
52  
Characteristics of the Day of Judgment, Paradise, and Hell
(18)
Chapter: Doing A Lot Of Good Deeds And Striving Hard In Worship
(18)
باب إِكْثَارِ الأَعْمَالِ وَالاِجْتِهَادِ فِي الْعِبَادَةِ ‏‏
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 ‏"‏ ‏.‏
Reference : Sahih Muslim 2819a
In-book reference : Book 52, Hadith 77
USC-MSA web (English) reference : Book 39, Hadith 6772
  (deprecated numbering scheme)