Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2621
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.