Sahih Muslim 1169
Sahih Muslim · Book 13: The Book of Fasting · The virtue of Lailat Al-Qadr and the Exhortation to seek it; When it is and the most likely times to seek it
'A'isha (Allah be pleased with her) and Ibn Numair reported Allah's Messenger (ﷺ) as saying:
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ ، حَدَّثَنَا ابْنُ نُمَيْرٍ ، وَوَكِيعٌ ، عَنْ هِشَامٍ ، عَنْ أَبِيهِ ، عَنْ عَائِشَةَ ، - رضى الله عنها - قَالَتْ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم قَالَ ابْنُ نُمَيْرٍ " الْتَمِسُوا - وَقَالَ وَكِيعٌ - تَحَرَّوْا لَيْلَةَ الْقَدْرِ فِي الْعَشْرِ الأَوَاخِرِ مِنْ رَمَضَانَ " .
Look for (and in the words of Waki, seek) Lailat-ul-Qadr in the last ten nights of Ramadan.