بُلُوغُ الْمَرَامِ
Bulugh al-Maram 380
The Book of Prayer
وَعَنْهَا قَالَتْ: { مِنْ كُلِّ اَللَّيْلِ قَدْ أَوْتَرَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -فَانْتَهَى وِتْرُهُ إِلَى اَلسَّحَرِ } مُتَّفَقٌ عَلَيْهِمَا . [1] .
Narrated ['Aishah (RA)]: Allah's Messenger (ﷺ) offered Witr prayer (on different nights) at various hours, extending (from the 'Isha' prayer) up to the last hour of the night. [Agreed upon].
Sunnah.com reference: Book 2, Hadith 284