Sunan an-Nasai Book 13, Hadith 1315

أَخْبَرَنَا مُحَمَّدُ بْنُ بَشَّارٍ، قَالَ حَدَّثَنَا يَحْيَى، عَنْ سَعِيدٍ، عَنْ قَتَادَةَ، عَنْ زُرَارَةَ بْنِ أَوْفَى، عَنْ سَعْدِ بْنِ هِشَامٍ، قَالَ قُلْتُ يَا أُمَّ الْمُؤْمِنِينَ أَنْبِئِينِي عَنْ وَتْرِ رَسُولِ اللَّهِ صلى الله عليه وسلم ‏.‏ قَالَتْ كُنَّا نُعِدُّ لَهُ سِوَاكَهُ وَطُهُورَهُ فَيَبْعَثُهُ اللَّهُ لِمَا شَاءَ أَنْ يَبْعَثَهُ مِنَ اللَّيْلِ فَيَتَسَوَّكُ وَيَتَوَضَّأُ وَيُصَلِّي ثَمَانِ رَكَعَاتٍ لاَ يَجْلِسُ فِيهِنَّ إِلاَّ عِنْدَ الثَّامِنَةِ فَيَجْلِسُ فَيَذْكُرُ اللَّهَ عَزَّ وَجَلَّ وَيَدْعُو ثُمَّ يُسَلِّمُ تَسْلِيمًا يُسْمِعُنَا ‏.‏

It was narrated that Sa'd bin Hisham said:"I said: 'O Mother of the believers! Tell me about the Witr of the Messenger of Allah (ﷺ).' She said: 'We used to prepare his siwak and water for purification, then Allah (SWT) would wake him when He willed to wake him at night. He would use the siwak and perform wudu, then pray eith rak'ahs; not sitting until the eighth rak'ah, when he would sit and remember Allah (SWT) and call upon Him. Then he would say the taslim loud enough for us to hear