The people complained to the Messenger of Allah ﷺ about the strain of keeping their elbows raised off the ground during prostration, so the Messenger of Allah ﷺ instructed them to seek support by resting them on their knees.
حدثنا قتيبة قال حدثنا يعقوب عن ابن عجلان عن سمي عن أبي صالح
شكا الناس إلى رسول الله ﷺ فتح ما بين المرفقين فأمرهم رسول الله ﷺ أن يستعينوا بالركب
No references are available.
Its isnad is sahih. It appeared earlier at 2/340 of the printed edition. It is found in Abu Dawud, and similarly worded as a marfu narration, "Seek help from riders" at 902, in the chapter on the permission for that, and in Tirmidhi 2/77 no. 286, and Hakim 1/229.