Retiring to a Mosque for Remembrance of Allah (I'tikaf)

Narrated Abu Sa`id Al-Khudri:
Allah's Apostle used to practice I`tikaf in the middle ten days of Ramadan and once he stayed in I`tikaf till the night of the twenty-first and it was the night in the morning of which he used to come out of his I`tikaf. The Prophet said, "Whoever was in I`tikaf with me should stay in I`tikaf for the last ten days, for I was informed (of the date) of the Night (of Qadr) but I have been caused to forget it. (In the dream) I saw myself prostrating in mud and water in the morning of that night. So, look for it in the last ten nights and in the odd ones of them." It rained that night and the roof of the mosque dribbled as it was made of leaf stalks of date-palms. I saw with my own eyes the mark of mud and water on the forehead of the Prophet (i.e. in the morning of the twenty-first)