Ubaydullah bin Abdullah bin Umar (may Allah have mercy on him) said that Ibn Umar (may Allah be pleased with them both) told them that a man called out to the Prophet (peace and blessings be upon him) while he (peace and blessings be upon him) was in the mosque, saying: "O Messenger of Allah! How should I make the night prayer odd (witr)?" He (peace and blessings be upon him) said: "Whoever prays at night should pray two rak'ahs at a time; if he fears that morning will come, he should make the prayer he has offered odd (witr), that is, pray one more rak'ah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4772