Musannaf Ibn Abi Shayba 2:20

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Abd al-Rahman ibn Yahya, from al-Hujay' (al-Rabi') ibn Qays, that Ali used to say: "The Adhan and the Iqamah are [to be performed] in pairs." He came upon a mu'adhdhin who was performing the Iqamah once [for each phrase], so he said: "Would you not make it in pairs, may you have no mother!"