It wasnarrated that Abu Ma'mar said:
"I said to Khabbab: 'How didyourecognize that the Messenger of Allah (ﷺ) was reciting in theZuhrand the 'Asr?' He said: 'From the movement of his beard.'" (RU)
English reference (RU): Vol. 1, Book 5, Hadith 826 (RU)
Arabic reference (RU): Book 5, Hadith 875 (RU)