Musannaf Ibn Abi Shayba 35:349

Ibn ‘Uyaynah narrated to us, from ‘Abdullah ibn Dinar, from Salim, from Ibn ‘Umar, from the Prophet (peace be upon him), who said: "The night prayer is two by two. So if you fear the morning, then perform Witr with one rak'ah, which will make Witr for you what has passed of your prayer."