Ma'mar narrates from al-Zuhri (may Allah have mercy on him) that when the Messenger of Allah (peace and blessings be upon him) began his sermon, they would turn their faces towards him (peace and blessings be upon him) until he (peace and blessings be upon him) finished the sermon.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5713