Sahih Bukhari Volume 3, Book 32, Hadith Number 235.
Narated By Abu Said Al-Khudri : Allah’s Apostle used to practice Itikaf (in the mosque) in the middle third of Ramadan and after passing the twenty nights he used to go back to his house on the 21st, and the people who were in Itikaf with him also used to go back to their houses. Once in Ramadan, in which he practiced Itikaf, he established the night prayers at the night in which he used to return home, and then he addressed the people and ordered them whatever Allah wished him to order and said, “I used to practice Itikaf for these ten days (i.e. the middle 113rd but now I intend to stay in Itikaf for the last ten days (of the month); so whoever was in Itikaf with me should stay at his place of seclusion. I have verily been shown (the date of) this Night (of Qadr) but I have forgotten it. So search for it in the odd nights of the last ten days (of this month). I also saw myself (in the dream) prostrating in mud and water.” On the night of the 21st, the sky was covered with clouds and it rained, and the rain-water started leaking through the roof of the mosque at the praying place of the Prophet. I saw with my own eyes the Prophet at the completion of the morning prayer leaving with his face covered with mud and water.