Sahih Muslim 1165
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
'Ibn 'Umar (Allah be pleased with both of them) reported Allah's Messenger (ﷺ) as saying:
وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ ، حَدَّثَنَا عَلِيُّ بْنُ مُسْهِرٍ ، عَنِ الشَّيْبَانِيِّ ، عَنْ جَبَلَةَ ، وَمُحَارِبٍ عَنِ ابْنِ عُمَرَ ، - رضى الله عنهما - قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " تَحَيَّنُوا لَيْلَةَ الْقَدْرِ فِي الْعَشْرِ الأَوَاخِرِ " . أَوْ قَالَ " فِي التِّسْعِ الأَوَاخِرِ " .
Seek the time of Lailat-ul-Qadr in the last (ten nights), or he said: in the last nine (nights).