(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.
Reference: Sahih Muslim 1165 e
In-book reference: Book 13, Hadith 271
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2621