Sayyiduna Uqbah bin Amr Abu Mas'ud al-Ansari (may Allah be pleased with him) said: the Messenger of Allah (peace and blessings be upon him) prayed witr sometimes at the beginning of the night, sometimes in the middle, and sometimes at the end.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14751