Musnad Ahmad 908:5

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Sulayman, who said: I heard ‘Umarah ibn ‘Umayr narrating from Abu Ma‘mar, who said: We asked Khabbab: "Did the Messenger of Allah, may Allah bless him and grant him peace, recite [the Qur’an] in the Dhuhr [prayer]?" He said: "Yes." He said: "And how did you know [that]?" He said: "By the movement of his beard."