Musannaf Ibn Abi Shayba 3:2886

Hushaym narrated to us, he said: Ibn ‘Awn informed us, from Ibrahim and Ibn Sirin, that he asked them about returning the greeting (salam) on Friday while the Imam is delivering the sermon. They both said: "It used to be said: Whoever says 'be silent' has engaged in vain talk (lagha)."