It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When someone tells his companion to be quiet while the imam is delivering the sermon, he has engaged in idle talk."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5826