Sahih Muslim Book 6, Hadith Number 2631.

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

‘Abdullah b. Unais reported Allah’s Messenger (may peace be upon him) as saying: I was shown Lailat-ul-Qadr; then I was made to forget it, and saw that I was prostrating in water and clay in the morning of that (night). He (the narrator) said: There was a downpour on the twenty-third night and the Messenger of Allah (may peace be upon him) led us in prayer, and as he went back, there was a trace of water and clay on his forehead and on his nose. He (the narrator) said: ‘Abdullah b. Unais used to say that it was the twenty-third (night).