Musnad Ahmad 980:21

Yahya ibn Hammad narrated to us, Abu 'Awanah informed us, from 'Ata' ibn al-Sa'ib, Salim al-Barrad narrated to us, he said: We entered upon Abu Mas'ud al-Ansari and asked him about the prayer. He said: "Shall I not pray for you as the Messenger of Allah, peace and blessings of Allah be upon him, used to pray?" He said: So he stood up and said the takbir and raised his hands. Then he bowed and placed his palms upon his knees and kept his armpits away from his sides. He said: Then he stood up until every part of him was settled. Then he prostrated and placed his palms and kept his armpits away from his sides. Then he raised his head until every part of him was settled. Then he prayed four rak'ahs like this.