سنن النسائي

It was narrated that Salim said: "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."