Musannaf Ibn Abi Shayba 3:4357

Waki’ narrated to us, he said: Shu’bah narrated to us, from Qatadah, from Khallas ibn ‘Amr al-Hajari, from ‘Ammar, who said: As for me, I perform Witr, and when I stand [for the night prayer], I pray two by two, and I leave my first Witr as it is.