Musnad Ahmad 1234:2

Abu Mu'awiyah narrated to us, he said: Al-A'mash narrated to us, from 'Umarah ibn 'Umayr, from Abu Ma'mar, he said: We said to Khabbab: "Did the Messenger of Allah, peace and blessings of Allah be upon him, recite in the Dhuhr and the 'Asr?" He said: "Yes." He said: So we said: "By what thing did you know that?" He said: So he said: "By the movement of his beard."