Musnad Abu Ya'la al-Mawsili 113:3

'Abd al-A'la ibn Hammad narrated to us, Yazid ibn Zuray' narrated to us, Khalid al-Hadhdha'—who used to be called Abu al-Manazil—narrated to us, from Abu Qilabah, from Anas, who said: Bilal was ordered to make the Adhan (call to prayer) double and the Iqamah (call to stand for prayer) single.