From Sahih Muslim (Hadith 2764):
Salim b. 'Abdullah b. 'Umar reported that his father said:I heard Allah's Messenger (ﷺ) 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)
Reference: Muslim 2764