Hadith 502720
Chapter 7: Tahajjud - كتاب صلاة الليل
وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ عَائِشَةَ، زَوْجَ النَّبِيِّ صلى الله عليه وسلم كَانَتْ تَقُولُ مَنْ خَشِيَ أَنْ يَنَامَ حَتَّى يُصْبِحَ فَلْيُوتِرْ قَبْلَ أَنْ يَنَامَ وَمَنْ رَجَا أَنْ يَسْتَيْقِظَ آخِرَ اللَّيْلِ فَلْيُؤَخِّرْ وِتْرَهُ .
Yahya related to me from Malik that he had heard that A'isha, the wife of the Prophet, may Allah bless him and grant him peace, used to say, "If someone fears that he will sleep through till the morning, let him pray the witr before he sleeps, and if some one hopes to wake for the last part of the night, let him delay his witr."
Reference Muwatta Malik 502720
USC-MSA web (English) reference : Book 7, Hadith 18