Ash-Shama'il Al-Muhammadiyah, Book 39, Hadith 248 (English) ⓘ

Mughirah bin Shu'bah Radiyallahu 'Anhu reports that Rasuluilah Sallallahu 'Alayhi Wasallam performed such lengthy nafl prayers, that his mubaarak legs became swollen. The Sahaabah said : "You undergo such great difficulties, where Allah had forgiven your past and the future sins. "Rasulullah Sallallahu 'Alayhi Wasallam said: "(When Allah Ta'aala has blessed me so much) should I not be a grateful servant?"

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، وَبِشْرُ بْنُ مُعَاذٍ، قَالا‏:‏ حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ زِيَادِ بْنِ عِلاقَةَ، عَنِ الْمُغِيرَةِ بْنِ شُعْبَةَ، قَالَ‏:‏ صَلَّى رَسُولُ اللهِ صلى الله عليه وسلم، حَتَّى انْتَفَخَتْ قَدَمَاهُ، فَقِيلَ لَهُ‏:‏ أَتَتَكَلَّفُ هَذَا، وَقَدْ غَفَرَ اللَّهُ لَكَ مَا تَقَدَّمَ مِنْ ذَنْبِكَ وَمَا تَأَخَّرَ‏؟‏ قَالَ‏:‏ أَفَلا أَكُونُ عَبْدًا شَكُورًا‏.‏