Musannaf Ibn Abi Shayba 2:25

Abu Bakr narrated to us, he said: Affan narrated to us, he said: Abd al-Wahid ibn Ziyad narrated to us, he said: al-Hajjaj ibn Artah narrated to us, he said: Abu Ishaq narrated to us, he said: The companions of Ali and the companions of Abdullah used to double the phrases of the Adhan and the Iqamah.