He attributed it to the Prophet ﷺ, who said, "When the reciter says ameen, then say ameen, for the angels say ameen. Whoever's ameen coincides with that of the angels, his past sins will be forgiven."
حدثنا سفيان بن عيينة قال حفظناه عن الزهري عن سعيد
يبلغ به النبي ﷺ إذا أمن القارئ فأمنوا فإن الملائكة تؤمن فمن وافق تأمينه تأمين الملائكة غفر له ما تقدم من ذنبه
Its isnad is sahih. Saeed is Ibn Musayab. The hadith is an abridgment of 7187, where it was given previously at length, narrated by Zuhri from Saeed bin Musayab and Abu Salama bin Abdul-Rahman, both from Abu Hurairah. His saying "he traced it back to the Prophet ﷺ" means he attributed the hadith to the Messenger of Allah ﷺ, so it carries the same force as saying "the Messenger of Allah said," and similar expressions.