Musannaf Ibn Abi Shayba 3:149

Abu Bakr narrated to us, he said: Abbad bin al-Awwam narrated to us, from Muhammad bin Amr, from Ali bin Yahya bin Khallad, from Rifa'ah bin Rafi', that the Prophet, may God bless him and grant him peace, said to a man: "When you face the Qibla, say 'Allahu Akbar' (Takbir), and recite what you wish. Then, when you intend to bow, place your palms upon your knees and settle into your bowing."