Abu Ayyub (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Pray five rak'ahs of witr; if you are not able to pray five, pray three rak'ahs of witr. If you are not able to pray three, pray one rak'ah of witr. Otherwise, perform it by gestures."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4778