Musannaf Ibn Abi Shayba 2:15

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Sulayman al-Taymi, he said: A man narrated to me in the Mosque of Kufa, from Ibn ‘Umar, he said: The Adhan is two by two, and the Iqamah is once. He said: Such was the Adhan of Bilal.