It was narrated that Sa'd bin Hish am said: "I asked 'Aishah: 'O Mother of the Believers! Tell me about the Witr of the Messenger of Allah SAW: She said: 'We used to keep his tooth stick and water for ablution ready for him. Allah would wake him as He willed to during the night, and he would use the tooth stick and perform ablution, then he would pray nine Rak'ah, during which he would not sit until the eighth Rak'ah. Then he would call upon his Lord and remember Allah and praise Him and supplicate to Him. Then he would get up without saying the Salam. Then he would stand up and pray the ninth Rak'ah.. Then he would sit and remember Allah and praise Him, and supplicate to his Lord and send blessing upon His Prophet. Then he would say Salam that we could hear. Then he would pray two Rak'ah after the Salam, while he was sitting down. That was eleven Rak' ah. When the Messenger of Allah SAW grew older and had gained weight, he would pray Witr with seven Rak'ah and then pray two more Rak'ah after he had said the Salam."
(Sahih).