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.'" (DE)
English reference (DE): Vol. 1, Book 5, Hadith 826 (DE)
Arabic reference (DE): Book 5, Hadith 875 (DE)