From Sahih Muslim (Hadith 2770):
Abu Sa'id al-Khudri (Allah be pleased with him) reported that the Messenger of Allah (ﷺ) 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
Reference: Muslim 2770