Musannaf Ibn Abi Shayba 2:11

Abu Bakr narrated to us, he said: Al-Thaqafi narrated to us, from Ayyub, from Abu Qilabah, he said: I think it is from Anas: He said: Bilal was ordered to perform the Adhan in pairs (shaf') and the Iqamah in single units (witr).