Musnad Ahmad 32:2064

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Ali ibn Zayd, who said: I heard Anas saying: "The caller to prayer would give the adhan during the time of the Messenger of Allah (peace and blessings of Allah be upon him), and we would think it was the iqamah because of the large number of people who would stand up to pray the two rak'ahs before the Maghrib prayer."