Musnad Ahmad 1018:92

'Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from Abu al-Za'ra', from Abu al-Ahwas, from one of the companions of the Prophet, may Allah bless him and grant him peace, who said: "The recitation of the Prophet, may Allah bless him and grant him peace, in the Dhuhr prayer used to be known by the movement of his beard."