Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "Do not pray three witr, and do not make it resemble Maghrib; rather, pray witr with five, seven, nine, eleven or more rak'ahs."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4816