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 (Subhanahu-wa-Taala) 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 (Subhanahu-wa-Taala) 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 (Subhanahu-wa-Taala) 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.'"

أَخْبَرَنَا زَكَرِيَّا بْنُ يَحْيَى، قَالَ حَدَّثَنَا إِسْحَاقُ، قَالَ أَنْبَأَنَا عَبْدُ الرَّزَّاقِ، قَالَ حَدَّثَنَا مَعْمَرٌ، عَنْ قَتَادَةَ، عَنْ زُرَارَةَ بْنِ أَوْفَى، أَنَّ سَعْدَ بْنَ هِشَامِ بْنِ عَامِرٍ، لَمَّا أَنْ قَدِمَ، عَلَيْنَا أَخْبَرَنَا أَنَّهُ، أَتَى ابْنَ عَبَّاسٍ فَسَأَلَهُ عَنْ وَتْرِ، رَسُولِ اللَّهِ ﷺ قَالَ أَلاَ أَدُلُّكَ أَوْ أَلاَ أُنَبِّئُكَ بِأَعْلَمِ أَهْلِ الأَرْضِ بِوَتْرِ رَسُولِ اللَّهِ ﷺ . قُلْتُ مَنْ قَالَ عَائِشَةُ . فَأَتَيْنَاهَا فَسَلَّمْنَا عَلَيْهَا وَدَخَلْنَا فَسَأَلْنَاهَا فَقُلْتُ أَنْبِئِينِي عَنْ وِتْرِ رَسُولِ اللَّهِ ﷺ . قَالَتْ كُنَّا نُعِدُّ لَهُ سِوَاكَهُ وَطَهُورَهُ فَيَبْعَثُهُ اللَّهُ عَزَّ وَجَلَّ مَا شَاءَ أَنْ يَبْعَثَهُ مِنَ اللَّيْلِ فَيَتَسَوَّكُ وَيَتَوَضَّأُ ثُمَّ يُصَلِّي تِسْعَ رَكَعَاتٍ وَلاَ يَقْعُدُ فِيهِنَّ إِلاَّ فِي الثَّامِنَةِ فَيَحْمَدُ اللَّهَ وَيَذْكُرُهُ وَيَدْعُو ثُمَّ يَنْهَضُ وَلاَ يُسَلِّمُ ثُمَّ يُصَلِّي التَّاسِعَةَ فَيَجْلِسُ فَيَحْمَدُ اللَّهَ وَيَذْكُرُهُ وَيَدْعُو ثُمَّ يُسَلِّمُ تَسْلِيمًا يُسْمِعُنَا ثُمَّ يُصَلِّي رَكْعَتَيْنِ وَهُوَ جَالِسٌ فَتِلْكَ إِحْدَى عَشْرَةَ رَكْعَةً يَا بُنَىَّ فَلَمَّا أَسَنَّ رَسُولُ اللَّهِ ﷺ وَأَخَذَ اللَّحْمَ أَوْتَرَ بِسَبْعٍ ثُمَّ يُصَلِّي رَكْعَتَيْنِ وَهُوَ جَالِسٌ بَعْدَ مَا يُسَلِّمُ فَتِلْكَ تِسْعًا أَىْ بُنَىَّ وَكَانَ رَسُولُ اللَّهِ ﷺ إِذَا صَلَّى صَلاَةً أَحَبَّ أَنْ يُدَاوِمَ عَلَيْهَا .