An Nasai

"It is established for you to hold the knees, so hold the knees."

" 'Umar said: 'The Sunnah is to hold the knees.'"

"We came to Abu Mas'ud and said to him: 'Tell us about the prayer of the Messenger of Allah (ﷺ).' He stood in front of us and said the takbir, then when he bowed he placed his palms on his knees and put his fingers lower than that, and he held his elbows out from his sides until every part of him had settled. Then he said: Sami' Allahu liman hamidah, Rabbana wa lakal-hamd (Allah hears those who praise Him, our Lord, and to You be the praise), then he stood up until every part of him had settled."

"Shall I not show you how I saw the Messenger of Allah (ﷺ) pray?" We said: "Yes." So he stood up and when he bowed, he placed his palms on his knees and put his fingers behind his knees, and held his arms out from his sides, until every part of him settled. Then he raised his head and stood up until every part of him settled. Then he prostrated and held his arms out from his sides, until every part of him settled. Then he sat up until every part of him settled. Then he prostrated again until every part of him settled. Then he did four rak'ahs like that. Then he said: "This is how I saw the Messenger of Allah (ﷺ) pray, and this is how he used to lead us in prayer."