Musnad Ahmad 32:2039

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, he said: I heard 'Amr ibn 'Amir al-Ansari, from Anas ibn Malik, who said: "When the mu'adhdhin would give the adhan, the companions of the Messenger of Allah (may Allah bless him and grant him peace) would stand up, hastening toward the pillars, until the Messenger of Allah (may Allah bless him and grant him peace) would come out, and they would be in that state—meaning the two rak'ahs before the Maghrib prayer. And there was not between the adhan and the iqamah except a short while."