Musnad Ahmad 908:27

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from 'Umarah ibn 'Umayr, from Abu Ma'mar, who said: We said to Khabbab: "Did the Messenger of Allah, may Allah bless him and grant him peace, recite in the Dhuhr and 'Asr [prayers]?" He said: "Yes." He said: So we said: "And by what thing did you know that?" He said: He replied: "By the movement of his beard."