Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2619
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).