From Sahih Muslim (Hadith 2763):
Salim reported on the authority of his father that a person saw Lailat-ul- Qadr on the 27th (of Ramadan). Thereupon Allah's Apostle (ﷺ) said:I see that your dreams agree regarding the last ten (nights of Ramadan). So seek it on an odd number (of these ten nights)
Reference: Muslim 2763