Sayyiduna Abu Sa'id (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) used to observe i'tikaf in the middle (ten days) of Ramadan. One year he (peace and blessings be upon him) observed i'tikaf, and when it was the twenty-first night, the night in which he (peace and blessings be upon him) would come out of his place of i'tikaf, he (peace and blessings be upon him) said: "Whoever has observed i'tikaf with me should also observe i'tikaf in the last ten. I was shown this night, then I was made to forget it. I saw myself prostrating in water and mud on the morning after it. So seek it in the last ten and in the odd nights." He said: Then it rained that night, and the (roof of the) mosque was made of a palm-frond shelter, and it kept (leaking). Sayyiduna Abu Sa'id (may Allah be pleased with him) said: My eyes saw the Messenger of Allah (peace and blessings be upon him) with traces of mud on his face and nose, and this was the twenty-first night.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8535