Musannaf Ibn Abi Shayba 3:4351

Waki’ narrated to us, from ‘Imran bin Hudayr, from Abu Mijlaz, that Usama bin Zayd and Ibn ‘Abbas said: "If you perform Witr at the beginning of the night, then you stand to pray, pray whatever seems good to you, then make it even with one rak’ah, then perform Witr."