Musnad Ahmad 457:19

Husayn ibn Ali narrated to us, from Za'idah, from 'Ata' ibn al-Sa'ib, from Salim Abu 'Abd Allah, who said: 'Uqbah ibn 'Amr said: "Shall I show you the prayer of the Messenger of Allah, may Allah bless him and grant him peace?" He said: "So he stood and said the takbir, then he bowed and kept his arms away from his sides, and placed his hands on his knees, and spread his fingers behind his knees, until every part of him was settled. Then he raised his head and stood, until every part of him was settled. Then he prostrated and kept his arms away from his sides, until every part of him was settled." He said: So he prayed four rak'ahs, then he said: "This is how I saw the Messenger of Allah, may Allah bless him and grant him peace, praying," or "This is how the Messenger of Allah, may Allah bless him and grant him peace, used to pray with us."