Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) used to perform i'tikaf in the middle ten days of the month. When twenty nights had passed and the twenty-first was approaching, he would return to his dwelling, and those who were in i'tikaf with him would also return. Then, in one month in which he was in i'tikaf, he stayed on during the night in which he used to return, and he addressed the people and commanded them with whatever Allah willed. Then he said: "I used to perform i'tikaf in these ten days; then it became clear to me that I should perform i'tikaf in these last ten days. So whoever has performed i'tikaf with me, let him spend the night in his place of i'tikaf." And he (peace and blessings be upon him) said: "I was shown this night, then I was made to forget it, so seek it in the odd nights of the last ten. And I saw myself prostrating in water and mud."
Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) said: Rain fell on us on the night of the twenty-first, and the mosque leaked at the place where the Messenger of Allah (peace and blessings be upon him) prayed. I looked at him when he had finished the Fajr prayer, and his face was wet with mud and water.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8589