Musnad Ahmad 3540

It was narrated that 'Abdullah (رضي الله عنه) said: We used to greet The Messenger of Allah (ﷺ) with salam when he was praying, and he would return the greeting. When we came back from being with the Negus, we greeted him with salam and he did not return the greeting. We said:O Messenger of Allah, we used to greet you when you were praying and you would return the greeting. He said: "There is sufficient preoccupation in the prayer."