بُلُوغُ الْمَرَامِ
Bulugh al-Maram 379
The Book of Prayer
وَعَنْهَا قَالَتْ: { كَانَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -يُصَلِّي مِنْ اَللَّيْلِ ثَلَاثَ عَشْرَةَ رَكْعَةً, يُوتِرُ مِنْ ذَلِكَ بِخَمْسٍ, لَا يَجْلِسُ فِي شَيْءٍ إِلَّا فِي آخِرِهَا. } [1] .
Narrated ['Aishah (RA)]: Allah's Messenger (ﷺ) used to pray thirteen Rak'at during the night, offering a Witr out of that with five and sitting only during the last of them. [Agreed upon].
Sunnah.com reference: Book 2, Hadith 283