Ibn Shihab (may Allah have mercy on him) narrates from Tha'labah bin Abi Malik (may Allah be pleased with him) that after the imam sits on the pulpit, voluntary prayer should end, and when he gives the sermon, conversation should end. The people would talk among themselves while Sayyiduna Umar (may Allah be pleased with him) was on the pulpit; when the caller fell silent, Sayyiduna Umar (may Allah be pleased with him) would stand, and then no one would speak until he had finished both sermons. When the prayer was established, Sayyiduna Umar (may Allah be pleased with him) would come down and the people would talk.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5686