Sahih al-Bukhari 611
صحيح البخاري#611SahihNarrated Abu Sa`id Al-Khudri:Allah's Messenger (ﷺ) said, "Whenever you hear the Adhan, say what the Mu'adh-dhin is saying
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 10:9