"[Some of] the Companions of the Prophet complained [to the Prophet] about the hardship of the prostration on them, when they were so spread out, so he said: 'Use your knees.'"  

اشْتَكَى بَعْضُ أَصْحَابِ النَّبِيِّ ﷺ إِلَى النَّبِيِّ ﷺ مَشَقَّةَ السُّجُودِ عَلَيْهِمْ إِذَا تَفَرَّجُوا فَقَالَ اسْتَعِينُوا بِالرُّكَبِ