Sahih al-Bukhari, Volume 2, Book 21, Hadith 230 (USC-MSA) ⓘ
Narrated Al-Mughira: The Prophet 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."
حَدَّثَنَا أَبُو نُعَيْمٍ، قَالَ حَدَّثَنَا مِسْعَرٌ، عَنْ زِيَادٍ، قَالَ سَمِعْتُ الْمُغِيرَةَ ـ رضى الله عنه ـ يَقُولُ إِنْ كَانَ النَّبِيُّ صلى الله عليه وسلم لَيَقُومُ لِيُصَلِّيَ حَتَّى تَرِمُ قَدَمَاهُ أَوْ سَاقَاهُ، فَيُقَالُ لَهُ فَيَقُولُ " أَفَلاَ أَكُونُ عَبْدًا شَكُورًا " .
also referenced under: Book 19, Hadith 10 (In-Book) & Hadith 1130 (DarusSalam)