Al-Aswad (may Allah have mercy on him) said: I heard Abdullah bin Mas'ud (may Allah be pleased with him) say that witr is between the two prayers, that is, Isha and Fajr; whenever you pray witr, it is good.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4528