Abd al-Rahman bin Jubayr bin Nufayr narrates from Sayyiduna Thawban (may Allah be pleased with him), the freed slave of the Prophet (peace and blessings be upon him), that he said: We were with the Prophet (peace and blessings be upon him) on a journey, and he (peace and blessings be upon him) said: "There is hardship in travel; when one of you prays witr, let him then offer two rak'ahs. If he wakes up at night, (well and good); otherwise these two rak'ahs will suffice him for the night."
(b) Imam al-Bayhaqi (may Allah have mercy on him) says: What is meant is either two rak'ahs after the witr or two rak'ahs before the witr.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4825