Hittan bin Abdullah (may Allah have mercy on him) says: I heard Sayyiduna Ali (may Allah be pleased with him) say: The witr is of three kinds: whoever wishes may pray witr in the first part of the night; then, if he wishes to pray (more), he keeps praying two rak'ahs at a time until morning. If he wishes, he may make his witr even by praying one rak'ah, then pray two rak'ahs at a time until morning, then pray witr. And whoever wishes may not pray the witr (early) and offer it in the last part of the night.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4850