Musannaf Ibn Abi Shayba 3:4404

Abu Bakr narrated to us, he said: Abd al-Salam ibn Harb narrated to us, from Yazid ibn Abd al-Rahman, from al-Hakam, he said: The Prophet, may Allah bless him and grant him peace, would sometimes gesture with his hand like this, and he would wipe his beard with his hand during the prayer.