Al-Rabi' bin Sulayman (may Allah have mercy on him) says that Imam al-Shafi'i (may Allah have mercy on him) informed him: Whoever prays witr in the first part of the night should pray two rak'ahs at a time until morning.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4849