Musnad Abu Ya'la al-Mawsili 117:178

Shayban narrated to us, Jarir narrated to us, Thabit narrated from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) would sometimes descend from the pulpit while the prayer had already been established, and a man would approach him and speak to him for a long time. Then, he would proceed to his place of prayer.