Musannaf Ibn Abi Shayba 2:65

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu'bah, from Mughirah, from Ibrahim, that he said regarding the Mu'adhdhin: He should keep his feet straight and face the Qiblah. When he says "Qad qamat al-salah" (the prayer has been established), he should turn his face to his right and his left.