Sahih al-Bukhari 1199
صحيح البخاري#1199SahihNarrated `Abdullah:We used to greet the Prophet (ﷺ) while he was praying and he used to answer our greetings. When we returned from An-Najashi (the ruler of Ethiopia), we greeted him, but he did not answer us (during the prayer) and (after finishing the prayer) he said, "In the prayer one is occupied (with a more serious matter)
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: 21:2