Musannaf Ibn Abi Shayba 2:60

Abu Bakr narrated to us, he said: Ibn al-Mubarak narrated to us, from Ma’mar, from Ayyub, from Ibn Sirin, who said: When the mu'adhdhin gives the adhan, he should face the qibla, and he used to dislike turning around in the minaret. And al-Hasan used to say: Face the qibla, and when he says "Hayya ‘ala al-salah" (come to prayer), he should turn around, and when he intends to say "Allahu Akbar" (God is the greatest), he should face the qibla.