From Sahih Muslim (Hadith 2765):
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: Muslim 2765