Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Do not pray three witr, (for) you would make it resemble the Maghrib prayer; pray witr with seven or five."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4815