It wasnarrated that Sa'd bin Hisham said:
"I asked 'Aishah: 'OMother of the Believers! Tell me about the Witr of the Messenger ofAllah (ﷺ).' She said: 'We used to keep his tooth stick andwater forablution ready for him. Allah would wake him as He willedto duringthe night, and he would use the tooth stick and performablution, thenhe would pray nine Rak'ah, during which he would notsit until theeighth Rak'ah. Then he would call upon his Lord andremember Allah andpraise Him and supplicate to Him. Then he wouldget up without sayingthe Salam. Then he would stand up and pray theninth Rak'ah. Then hewould sit and remember Allah and praise Him,and supplicate to hisLord and send blessing upon His Prophet. Thenhe would say Salam thatwe could hear. Then he would pray two Rak'ahafter the Salam, while hewas sitting down. That was eleven Rak'ah.When the Messenger of Allah(ﷺ) grew older and had gained weight,he would pray Witr with sevenRak'ah and then pray two more Rak'ahafter he had said the Salam.'"
English reference: Vol. 1, Book 5, Hadith 1191
Arabic reference: Book 5, Hadith 1247