Musannaf Ibn Abi Shayba 3:153

Abu Bakr narrated to us, he said: 'Abdah and Waki' narrated to us, from Isma'il, from al-Zubayr ibn 'Adi, from Mus'ab ibn Sa'd, who said: I performed a rak'ah beside my father, and I placed my hands between my knees. So Sa'd struck my hands, then he said: "We used to do this, then we were commanded to [place them on] the knees."