Musannaf Ibn Abi Shayba 3:585

Abu al-Ahwas narrated to us, from ‘Ata’ ibn al-Sa’ib, from Salim al-Barrad, who said: We came to Abu Mas’ud al-Ansari in his house and said to him, "Tell us about the prayer of the Messenger of Allah (may Allah bless him and grant him peace)." So he stood up to pray in front of us. When he bowed, he placed his palms on his knees and positioned his fingers below that, and he kept his elbows away from his sides until everything of him was level. Then he raised his head and said, "Allah hears those who praise Him," and he stood until everything of him was level. Then he prostrated and did the same. He prayed two rak'ahs, and when he finished them, he said, "This is how I saw the Messenger of Allah (may Allah bless him and grant him peace) praying."