Musannaf Ibn Abi Shayba 2:131

Waki’ narrated to us, he said: Al-A’mash narrated to us, from ‘Amr ibn Murrah, from Ibn Abi Layla, he said: The companions of Muhammad, may Allah bless him and grant him peace, narrated to us that Bilal would perform the Adhan in pairs and the Iqamah in pairs, and he would sit for a sitting.