Musannaf Ibn Abi Shayba 2:235

Abu Bakr narrated to us, he said: Ibn Mahdi narrated to us, from Sufyan, from al-Zubayr ibn ‘Adi, from a man, from Ibn ‘Umar, that he said to a man: "What is your work?" He said: "The Adhan." He said: "An excellent work. Everything that hears you will bear witness for you."