كتاب الصيام
13
The Book of Fasting
(40)
Chapter: The virtue of Lailat Al-Qadr and the Exhortation to seek it; When it is and the most likely times to seek it
(40)
باب فَضْلِ لَيْلَةِ الْقَدْرِ وَالْحَثِّ عَلَى طَلَبِهَا وَبَيَانِ مَحَلِّهَا وَأَرْجَى أَوْقَاتِ طَلَبِهَا

Ibn Umar (Allah be pleased with them) reported Allah's Messenger (ﷺ) as saying:

Seek it (Lailat-ul-Qadr) in the last (ten nights). If one among you shows slackness and weakness (in the earlier part of Ramadan), it should not be allowed to prevail upon him in the last week.
وَحَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ عُقْبَةَ، - وَهُوَ ابْنُ حُرَيْثٍ - قَالَ سَمِعْتُ ابْنَ عُمَرَ، - رضى الله عنهما - يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ الْتَمِسُوهَا فِي الْعَشْرِ الأَوَاخِرِ - يَعْنِي لَيْلَةَ الْقَدْرِ - فَإِنْ ضَعُفَ أَحَدُكُمْ أَوْ عَجَزَ فَلاَ يُغْلَبَنَّ عَلَى السَّبْعِ الْبَوَاقِي ‏"‏ ‏.‏
Reference : Sahih Muslim 1165 e
In-book reference : Book 13, Hadith 271
USC-MSA web (English) reference : Book 6, Hadith 2621
  (deprecated numbering scheme)