Musnad Ahmad 908:9

Waki' narrated to us, Al-A'mash narrated to us, from 'Umarah ibn 'Umayr, from Abu 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."