Masruq narrates from Sayyidah Aishah (may Allah be pleased with her): "The Prophet (peace and blessings be upon him) prayed witr in every part of the night, and the latest time of his (peace and blessings be upon him) witr reached the pre-dawn (sahar)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4833