Musnad Ahmad 1065:6

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, 'Asim ibn 'Umar ibn Qatadah al-Ansari narrated to me, from Mahmud ibn Labid, brother of the Banu 'Abd al-Ashhal, who said: The Messenger of Allah (peace and blessings of Allah be upon him) came to us and led us in the Maghrib prayer in our mosque. When he had finished it with the taslim, he said: "Perform these two rak'ahs in your houses," referring to the voluntary prayer after Maghrib.