Sahih Muslim Book 6, Hadith Number 2625.

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

Abu Sa’id al-Khudri (Allah be pleased with him) reported that Allah’s Messenger (may peace be upon him) spent in devotion (in i’tikaf) the middle ten nights of the month of Ramadan, and when twenty nights were over and it was the twenty-first night, he went back to his residence and those who were along with him also returned (to their respective residences).He spent one month in devotion. Then he addressed the people on the night he came back (to his residence) and commanded them as Allah desired (him to command) and then said: I used to devote myself (observe i’tikaf) during these ten (nights).Then I started devoting myself in the last ten (nights).And he who desires to observe i’tikaf along with me should spend the night) at his place of i’tikaf. And I saw this night (Lailat-ul-Qadr) but I forgot it (the exact night); so seek it ;In the last ten nights on odd numbers. I saw (the glimpses of that dream) that I was prostrating in water and mud. Abu Sa’id al-Khudri said: It rained on the twenty-first night and the water dripped (from the roof) of the mosque at the place where the Messenger of Allah (may peace be upon him) observed prayer. I looked at him and as he completed the dawn prayer, (I found) his face was wet with mud and water.