It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever speaks during the Imam's sermon on Friday, his example is like that of a donkey carrying a heavy load, and whoever says to the one speaking, 'Be quiet,' he has no Friday (i.e., his Friday is invalidated)."
Hadith Referenceمسند احمد / ومن مسند بني هاشم / 2033