Sahih Muslim Book 6, Hadith Number 2619.

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

Salim reported on the authority of his father that a person saw Lailat-ul-Qadr on the 27th (of Ramadan).Thereupon Allah’s Apostle (may 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).