Musannaf Ibn Abi Shayba 3:1258

Abu Bakr narrated to us, he said: Abu Mu'awiyah and Waki' narrated to us, from al-A'mash, from 'Umarah ibn 'Umayr, from Abu Ma'mar, he 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 the 'Asr?" He said: "By the movement of his beard." And Abu Mu'awiyah said: "By his two jaws."