Musnad Ahmad 3861

It was narrated that Ibraheem said: 'Abdullah (رضي الله عنه) said: We used to greet The Messenger of Allah (ﷺ) with salam when he was praying (and he would return our greeting), until we came back from being with the Negus, we grected him with salam (whilst he was praying) and he did not return the greeting. And he said: “There is sufficient preoccupation in the prayer."