It was narrated from Zurarah bin Awfa:
When Sa'd bin Hisham bin Amir came to visit us, he told us that he came to Ibn Abbas and asked him about the witr of the Messenger of Allah (ﷺ). He said: 'Shall I not tell you of the most knowledgeable person on Earth about the witr of the Messenger of Allah (ﷺ)?' I said: 'Who?' He said: 'Aishah.' So we went to her and greeted her with Salam and entered and asked her. I said: 'Tell me about the witr of the Messenger of Allah (ﷺ).' She said: 'We used to prepare for him his siwak and water for wudu, then Allah (SWT) would wake him when He willed to wake him at night. He would clean his teeth and perform wudu, then he would pray nine rak'ahs, during which he would not sit until the eighth. Then he would praise Allah (SWT) and remember Him and supplicate, then he would get up and not say the taslim. Then he would pray the ninth, then sit and praise Allah (SWT) and remember Him and supplicate, then he would say a taslim that we could hear. Then he prayed two rak'ahs sitting, and that was eleven rak'ahs, O my son. When the Messenger of Allah (ﷺ) grew older and put on weight, he prayed witr with seven, then he prayed two rak'ahs sitting after saying the taslim, and that was nine, O my son. And when the Messenger of Allah (ﷺ) offered a prayer, he liked to persist in offering it.'