Sayyiduna Thawban (may Allah be pleased with him) narrates: We were travelling with the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) said: "This journey is a source of hardship and burden. When any of you has prayed the Witr, he should pray two rak'ahs. If he wakes up, well and good; otherwise those two will count for him."
Al-Tabarani recorded it in al-Mu'jam al-Kabir and al-Mu'jam al-Awsat; its chain contains Abdullah bin Salih, the scribe of al-Layth, who has been criticized.