Sayyiduna Thawban (may Allah be pleased with him) narrates: We were with the Messenger of Allah (peace and blessings be upon him) on a journey, and he (peace and blessings be upon him) said: "This journey is one of hardship and burden. So when one of you has prayed witr, let him pray two (voluntary) rak'ahs; if he wakes up (at night), well and good; otherwise these will suffice for him."
Hadith Referenceالمعجم الكبير للطبراني / باب الثاء / 1397