From Sahih Muslim (Hadith 2761):
Ibn 'Umar (Allah be pleased with them) reported that some persons among the Companions of the Messenger of Allah (ﷺ) were shown Lailat- ul-Qadr while sleeping in the last week (of Ramadan). Thereupon Allah's Messenger (ﷺ) 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)
Reference: Muslim 2761