Musannaf Ibn Abi Shayba 2:19

Abu Bakr narrated to us, he said: Abda narrated to us, from Isma'il, from Abu al-Muthanna, that Ibn Umar used to order the mu'adhdhin to make the adhan even (double) and the iqamah odd (single), so that the passerby might distinguish the adhan from the iqamah.