Musannaf Ibn Abi Shayba 3:3556

Waki’ narrated to us, he said: Musa bin Ubaydah narrated to us, from Ayyub bin Khalid, from Ibn Umar, he said: "Whoever prays four [rak'ahs] after the Maghrib [prayer] is like one who performs a follow-up [act of worship] after a follow-up."