Musnad Ahmad 27:1153

'Abd al-Rahman ibn Mahdi narrated to us, Shu'bah narrated to us, from Abu Ja'far, I heard Abu al-Muthanna narrating from Ibn 'Umar, who said: "The Adhan during the time of the Messenger of Allah, peace and blessings of Allah be upon him, was two by two. And the Iqamah was once, except that when the Mu'adhdhin said: 'Qad qamat al-salah' (the prayer has been established), he would say: 'Qad qamat al-salah' twice."