Sahih al-Bukhari 1130
Book 19: Prayer at Night (Tahajjud) · Narrated by Al-Mughira
Arabic
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 ".
English translation
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, Book of Prayer at Night (Tahajjud). English translation by Dr. Muhammad Muhsin Khan.