He prayed behind the Messenger of Allah ﷺ and heard him say "Ameen" aloud, then give the salutation to his right and to his left until I could see the whiteness of his cheek.
حدثنا مخلد بن خالد الشعيري حدثنا ابن نمير حدثنا علي بن صالح عن سلمة بن كهيل عن حجر بن عنبس
أنه صلى خلف رسول الله ﷺ فجهر بآمين وسلم عن يمينه وعن شماله حتى رأيت بياض خده
[Sahih] Bayhaqi narrated it in Khilafiyyat (folio 51/1a) through Abu Dawud's route, and in his version it is Ala bin Salih, which is correct. The chain is hasan, and the hadith has corroborating narrations. Ala bin Salih was deemed reliable by Ibn Main and the majority of scholars, so he is hasan al-hadith.