Musannaf Ibn Abi Shayba 3:2931

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu'bah, from al-A'mash, from Abu Salih, from Abu Hurayrah, who said: If he says "shush" (soh) on the day of Friday while the Imam is delivering the sermon, he has engaged in idle talk (lagha).