Musannaf Ibn Abi Shayba 3:2936

Yahya ibn Salim al-Ta'ifi narrated to us, from Hisham ibn Urwah, who said: I reached my father and those who have passed away from among those whose [knowledge] is accepted and from whom [knowledge] is taken; they did not see any harm in speaking from the time the Imam descends from the pulpit until he enters into the prayer.