Musnad Ahmad 27:93

Sufyan narrated to us, from al-Zuhri, from Salim, from his father: "I saw the Messenger of Allah, may Allah bless him and grant him peace, when he began the prayer, he would raise his hands until they were level with his shoulders. And when he intended to bow, and after he raised his head from bowing—and Sufyan said: 'Once, and when he raised his head'—and the most he would say is: 'And after he raised his head from bowing,' and he would not raise them between the two prostrations."