Narrated by Aisha (may Allah be pleased with her), she states that the Messenger of Allah (peace be upon him) performed Witr in every part of the night, and in the latter part of his life, his Witr would (usually) end at the time of Sahar (pre-dawn).
Hadith Referenceمسند الإمام الشافعي / كتاب الصلاة / 389
Hadith Takhrijاخرجه البخاري الوتر، باب ساعات الوتر (996) ومسلم صلاة المسافرين، باب صلاة الليل وعدد ركعات النبي في الليل...... الخ (745).