Tha'labah bin Abi Malik al-Qurazi (may Allah have mercy on him), who met Umar bin al-Khattab (may Allah be pleased with him), says: We used to keep talking among ourselves while Umar (may Allah be pleased with him) was on the minbar, until the mu'adhdhin finished his call to prayer and Umar (may Allah be pleased with him) began the sermon. When Umar (may Allah be pleased with him) began the sermon, our talk would end and we would fall silent, and then no one would speak until the imam finished the sermon.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5717