Tha’laba bin Abi Malik (may Allah have mercy on him) narrates that the Imam sitting for the sermon ends the prayer, and the Imam’s sermon prevents conversation, and people used to talk on Friday while ‘Umar (may Allah be pleased with him) was seated on the pulpit. When the muezzin finished the call to prayer and ‘Umar (may Allah be pleased with him) stood up for the sermon, then none of them would speak until both sermons were finished, and when the prayer was established and ‘Umar (may Allah be pleased with him) descended from the pulpit, they would converse.
Hadith Referenceمسند الإمام الشافعي / كتاب الجمعة / 426