Musannaf Ibn Abi Shayba 3:6416

Waki' narrated to us, he said: Al-A'mash narrated to us, from 'Umarah, from Ibn Ma'mar, who said: We said to Khabbab, "By what thing did you used to know the recitation of the Messenger of Allah (may Allah bless him and grant him peace) in the Dhuhr and 'Asr [prayers]?" He said: "By the movement of his beard."