Musannaf Ibn Abi Shayba 3:2925

Abu Dawud al-Tayalisi narrated to us, from Ibrahim ibn Nafi’, from Sa’id ibn Abi Hind, from Humayd ibn ‘Abd al-Rahman, who said: If a man says on Friday while the Imam is delivering the sermon, "Be silent," then he has engaged in vain talk (lagha).