Musnad Abu Ya'la al-Mawsili 51:7

Muhammad ibn al-Khattab narrated to us, Mu'ammal narrated to us, Shu'bah narrated to us, Husayn narrated to us, from 'Abd al-Malik, from 'Amr ibn Hurayth, who said: The Prophet, peace and blessings of Allah be upon him, would sometimes touch his beard during the prayer.