Sayyiduna Abdullah (may Allah be pleased with him) narrates that we used to greet the Prophet (peace be upon him) with salam while he was praying, and he (peace be upon him) would respond to our salam. So when we returned from Najashi (after the migration to Abyssinia), we greeted the Prophet (peace be upon him) with salam (while he was in prayer), but he (peace be upon him) did not respond to our salam. We said, "O Messenger of Allah (peace be upon him), we used to greet you with salam during prayer and you would respond to our salam. (Why did you not respond now?)" The Prophet (peace be upon him) replied: "There is engagement in prayer."