Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyiduna Thawban (may Allah be pleased with him), he says: We were on a journey with the Messenger of Allah (peace and blessings be upon him), and he said: "Since this journey is a cause of hardship and difficulty, everyone should perform two units of voluntary prayer after Witr. If (he) wakes up (to stand in prayer), then fine, otherwise these two units will suffice for him."