Sahih al-Bukhari, Book 81, Hadith 60 (In-Book) ⓘ

Narrated Al-Mughira bin Shu`ba: The Prophet 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)?"

حَدَّثَنَا خَلاَّدُ بْنُ يَحْيَى، حَدَّثَنَا مِسْعَرٌ، حَدَّثَنَا زِيَادُ بْنُ عِلاَقَةَ، قَالَ سَمِعْتُ الْمُغِيرَةَ بْنَ شُعْبَةَ، يَقُولُ كَانَ النَّبِيُّ صلى الله عليه وسلم يُصَلِّي حَتَّى تَرِمَ ـ أَوْ تَنْتَفِخَ ـ قَدَمَاهُ فَيُقَالُ لَهُ، فَيَقُولُ ‏ "‏ أَفَلاَ أَكُونُ عَبْدًا شَكُورًا ‏" ‏‏.‏