Musannaf Ibn Abi Shayba 2:35

Abu Bakr narrated to us, he said: Muhammad ibn Fudayl narrated to us, from Zayd, from Abu Sadiq, that he used to make the end of his Adhan: "La ilaha illa Allah, wa Allahu Akbar." And he said: "This is how the end of Bilal's Adhan was."