Salim reported on the authority of his father that a person saw Lailat-ul- Qadr on the 27th (of Ramadan). Thereupon Allah’s Apostle (peace be upon him) 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).
(Sahih Muslim, Book 13, Hadith 269)