Musannaf Ibn Abi Shayba 3:5001

Abu Bakr narrated to us, he said: Sharik narrated to us, from Asim, from Zirr, he said: I saw Abd al-Rahman ibn Awf and Ubayy ibn Ka'b, when the caller to prayer (mu'adhdhin) gave the call for Maghrib, they would stand and pray two rak'ahs.