Musnad Abu Ya'la al-Mawsili 131:10

Abdullah ibn Umar ibn Aban al-Kufi narrated to us, Muhammad ibn Fudayl narrated to us, from al-Mukhtar ibn Fulful, who said: I asked Anas ibn Malik about the prayer after the Asr [prayer]. He said: Umar ibn al-Khattab used to strike [people] for the prayer after Asr. He said: We used to pray two rak'ahs during the time of the Messenger of Allah (peace and blessings of Allah be upon him) after the sun had set, before the Maghrib prayer. I said: Did the Messenger of Allah (peace and blessings of Allah be upon him) pray them? He said: He used to see us praying them, and he neither commanded us [to do so] nor forbade us.