Sahih Muslim Book 6, Hadith Number 2621.

Chapter : Excellence of Lailat-ul Qadar and exhortation to find it.

Ibn Umar (Allah be pleased with them) reported Allah’s Messenger (may peace be upon him) 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.