Musnad Ahmad 32:1115

'Abd al-Wahid Abu 'Ubaydah al-Haddad narrated to us, al-Mu'alla ibn Jabir—meaning al-Luqayti—narrated to us, he said: Musa ibn Anas ibn Malik narrated to me, from his father, who said: "When the mu'adhdhin would stand and call the adhan for the Maghrib prayer in the mosque in Medina, whoever wished would stand and pray until the prayer was established, and whoever wished would perform two rak'ahs and then sit. And that was in the presence of the Prophet, may Allah bless him and grant him peace."