Ayesha said, "He would pray nine units of prayer, sitting only during the eighth. He would remember Allah, praise Him, and supplicate to Him, then rise without saying the closing salutation. He would stand and pray the ninth, then sit and remember Allah, praise Him, and supplicate to Him, then give the closing salutation loudly enough for us to hear. Then he would pray two more units while seated after the salutation. That makes eleven units, my son.When the Prophet of Allah ﷺ grew older and gained weight, he would pray seven units as his witr, doing with the two seated units as he had done before. That makes nine, my son."
قالت عائشة: يصلي تسع ركعات، لا يجلس فيها إلا في الثامنة فيذكر الله، ويحمده ويدعوه. ثم ينهض ولا يسلم. ثم يقوم فيصلي التاسعة. ثم يقعد فيذكر الله ويحمده ويدعوه. ثم يسلم تسليما يسمعنا. ثم يصلي ركعتين بعد ما يسلم وهو قاعد. فتلك إحدى عشرة ركعة يا بني. فلما أسن نبي الله ﷺ، وأخذه اللحم، أوتر بسبع. وصنع في الركعتين مثل صنيعه الأول، فتلك تسع يا بني.
Muslim narrated it in "The Prayer of the Traveler" (746), from Muhammad bin Muthanna Anazi, who said, Muhammad bin Abu Adi narrated to us, from Saeed, from Qatada, from Zurara, from Saad bin Hisham bin Amir, and he mentioned it in a long hadith that was already mentioned in the comprehensive chapter on the night prayer of the Messenger of Allah ﷺ.