They could tell that the Prophet ﷺ was reciting during the Zuhr prayer by the movement of his beard.
كانت تعرف قراءة النبي ﷺ في الظهر بتحريك لحيته.
Imam Ahmad narrated it (23153) from Abdul-Rahman bin Mahdi, from Sufyan, from Abu Zaraa, from Abu Ahwas, from one of the companions of the Prophetﷺ. Ibn Abu Shayba also narrated it (1/362) from Waki, from Sufyan, in the same manner.Busiri cited it in Ithaf Mahra (1851) from Ibn Abu Shayba and Ahmad, and said, "This isnad has trustworthy men, and Abu Zaraa is Amr bin Amr." End of quote.I say, it is as he said, for Amr bin Amr bin Malik, Abu Zaraa, even though the two sheikhs did not report from him, is nonetheless trustworthy. Ibn Main deemed him trustworthy, and Ahmad said, "A trustworthy sheikh." Ajli and Nasai also deemed him trustworthy in his surname entries, and Ibn Abdul-Barr said, "They agreed that he is trustworthy." However, Abu Hatim said of him, "Truthful."