(6) باب قِيَامِ النَّبِيِّ صلى الله عليه وسلم حَتَّى تَرِمَ قَدَمَاهُ
Narrated Al-Mughira:
The Prophet (ﷺ) 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."
Reference: Sahih al-Bukhari 1130
In-book reference: Book 19, Hadith 10
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 21, Hadith 230