Ishaq bin Mansur al-Kausaj إسحاق بن منصور بن بهرام - الكوسج · The Book on Al-Witr

"The night prayer of Allah's Messenger was thirteen Rak'ah, five of which were his Witr, not sitting in any of them except at the end of them. When the Mu'adh-dhin called the Adhan he would stand to perform two light (Rak'ah)."