Musannaf Ibn Abi Shayba 3:3014

Narrated to us Kathir ibn Hisham, from Ja'far ibn Burqan, from Maymun, who said: It used to be in Medina, when the caller to prayer (mu'adhdhin) would give the call on Friday, they would call out in the marketplaces: "Selling is forbidden, selling is forbidden."