Musannaf Ibn Abi Shayba 3:4356

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu'bah, from Ibrahim ibn al-Muhajir, from Kulayb al-Jarmi, from Sa'd, who said: "As for me, when I have performed the Witr prayer, then I stand and pray two rak'ahs by two rak'ahs."