Sahih Muslim 1167c

Abu Sa'id al-Khudri (Allah be pleased with him) reported that the Messenger of Allah (ﷺ) observed i'tikaf (confined himself for devotion and prayer) in the first ten (days) of Ramadan; he then observed i'tikaf in the middle ten (days) in a Turkish tent with a mat hanging at its door. He (the Holy Prophet) took hold of that mat and placed it in the nook of the tent. He then put his head out and talked with people and they came near him, and he (the Holy Prophet) said:

I observed i'tikaf in the first ten (nights and days) in order to seek that night (Lailat-ul-Qadr). I then observed i'tikaf in the middle ten days. Then (an angel) was sent to me and I was told that this (night) is among the last ten (nights). He who among you likes to observe i'tikaf should do so; and the people observed it along with him, and he (the Holy Prophet) said: That (Lailat-ul-Qadr) was shown to me on an odd (night) and I (saw in the dream) that I was prostrating in the morning in clay and water. So in the morning of the twenty-first night when he (the Holy Prophet) got up for dawn (prayer). there was a rainfall and the mosque dripped, and I saw clay and water. When he came out after completing the morning prayer (I saw) that his forehead and the tip of his nose had (traces) of clay and water, and that was the twenty-first night among the last ten (nights).