Sahih Muslim Book 6, Hadith Number 2626.
Chapter : Excellence of Lailat-ul Qadar and exhortation to find it.
Abu Sa’id al-Khudri (Allah be pleased with him) reported that the Messenger of Allah (may peace be upon him) devoted (himself to prayer) in the middle (ten nights) of Ramadan. The rest of the Hadith is the same except for these words: “That he adhered to his place of i’tikaf and his forehead was besmeared with mud and water.”