Sahih Muslim Book 6, Hadith Number 2617.

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

Ibn ‘Umar (Allah be pleased with them) reported that some persons among the Companions of the Apostle of Allah (may peace be upon him) were shown Lailat-ul-Qadr while sleeping in the last week (of Ramadan). Thereupon Allah’s Messenger (may peace be upon him) said: I see that your dreams agree regarding the last week; so he who wants to seek it should seek it in the last week (during the night).