Musnad Ahmad 908:10

'Abd al-Rahman narrated to us, from Sufyan, from Sulayman al-A'mash, from 'Umarah, from Abu Ma'mar, from Khabbab, who said: It was said to him: "Did the Prophet, peace and blessings of Allah be upon him, recite in the Dhuhr and 'Asr prayers?" He said: "Yes." It was said: "By what means did you know that?" He said: "By the movement of his beard."