Hadithcore
Mishkat al-Masabih, 1256

‘A’isha said that God’s Messenger used to pray thirteen <i>rak'as</i> during the night, observing a <i>witr</i> out of that with five, and sitting only during the last of them. (Bukhari and Muslim.)

وَعَنْ عَائِشَةَ رَضِيَ اللَّهُ عَنْهَا قَالَتْ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُصَلِّي مِنَ اللَّيْلِ ثَلَاثَ عَشْرَةَ رَكْعَةً يُوتِرُ مِنْ ذَلِكَ بِخَمْسٍ لَا يَجْلِسُ فِي شَيْء إِلَّا فِي آخرهَا "

References1 variant
In-Book Reference
Book 4, Hadith 671