Sahih Muslim Book 6, Hadith Number 2620.

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

Salim b. ‘Abdullah b. ‘Umar reported that his father said: I heard Allah’s Messenger (may peace be upon him) as saying: So far as Lailat-ul-Qadr is concerned, some persons among you have seen it (in a dream) in the first week and some persons among you have been shown that it is in the last week; so seek it in the last ten (nights).