Musannaf Ibn Abi Shayba 13:2410

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Yunus, from al-Hasan, he said: When he begins, he should bear witness (i.e., recite the Adhan), and when he is called, he should establish it (i.e., recite the Iqamah).