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."  

سَمِعْتُ الْمُغِيرَةَ ؓ يَقُولُ إِنْ كَانَ النَّبِيُّ ﷺ لَيَقُومُ لِيُصَلِّيَ حَتَّى تَرِمُ قَدَمَاهُ أَوْ سَاقَاهُ فَيُقَالُ لَهُ فَيَقُولُ أَفَلاَ أَكُونُ عَبْدًا شَكُورًا