Sunan an-Nasa'i, Book 20, Hadith 84 (In-Book) ⓘ
It was narrated that Aishah said: "The Messenger of Allah prayed witr at the beginning (of the night) and at the end, and in the middle. And toward the end of his life, he settled on performing witr at the end of the night."
أَخْبَرَنَا إِسْحَاقُ بْنُ مَنْصُورٍ، قَالَ حَدَّثَنَا عَبْدُ الرَّحْمَنِ، عَنْ سُفْيَانَ، عَنْ أَبِي حَصِينٍ، عَنْ يَحْيَى بْنِ وَثَّابٍ، عَنْ مَسْرُوقٍ، عَنْ عَائِشَةَ، قَالَتْ أَوْتَرَ رَسُولُ اللَّهِ صلى الله عليه وسلم مِنْ أَوَّلِهِ وَآخِرِهِ وَأَوْسَطِهِ وَانْتَهَى وَتْرُهُ إِلَى السَّحَرِ .
also referenced under: Hadith 1681 (Reference) & Volume 2, Book 20, Hadith 1682 (DarusSalam)