Sahih Muslim Book 6, Hadith Number 2622.
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: He who is anxious to seek it (Lailat-ul-Qadr) should seek it in the last ten (nights of Ramadan).