Abu Sa’id Al Khudri :
The Messenger of Allah (ﷺ) used to spend the middle ten days of Ramadan in retirement and devotion (i'tikaf) in the mosque. One year he had retirement and devotion in the mosque (as usual); when the twenty-first night came, and this night when he used to come out his devotion in the mosque, he said: He who has engaged himself in devotion along with me should do so during the last ten days; I saw that night, that was caused to forget it, but I have seen myself prostrating in water and mud on the morning following (that night), so seek it in the last ten days and seek it every night with an odd number. Abu sa'id said: Rain fell that night, the mosque that was thatched building dripped, and my eyes saw the Messenger of Allah (ﷺ) with the traces of water and mud, on his forehead on the morning following the twenty-first night.