Hisham bin Urwah narrates from his father that Sayyidah Aishah (may Allah be pleased with her) said: The Messenger of Allah (peace and blessings be upon him) used to pray thirteen rak'ahs, five of which were witr. The Prophet (peace and blessings be upon him) would not sit during those five, and he would say the salam in the last rak'ah.
(b) In the narration of Abu Bakr: The night prayer of the Prophet (peace and blessings be upon him) was thirteen rak'ahs, five of which were witr, and he would sit only in the last rak'ah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4800