Sunan al-Shafi'i

It is narrated from Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) used to observe i'tikaf during the middle ten days of Ramadan. After spending the nights, when the twenty-first night would come, he would return home in the evening, and those who were observing i'tikaf with him would also return. But once, on the twenty-first night, he remained in the mosque, although it was his habit to return home. Then the Messenger of Allah (peace and blessings be upon him) delivered a sermon to the people and commanded whatever Allah, the Mighty and Majestic, willed. Then he said: "I used to observe i'tikaf in the middle ten days, but now it has become clear that i'tikaf should be observed in the last ten days. Therefore, whoever has observed i'tikaf with me should remain in i'tikaf. I was shown Laylat al-Qadr, then it was made to be forgotten. Seek it in the last ten nights and seek it in the odd nights. I saw myself (in a dream) prostrating in mud." Sayyiduna Abu Sa'id (may Allah be pleased with him) also said that on that night clouds appeared in the sky, it rained, and water dripped from the roof onto the prayer mat of the Prophet (peace and blessings be upon him). This happened on the twenty-first night. I saw with my own eyes that when the Messenger of Allah (peace and blessings be upon him) led us in the morning prayer and turned his blessed face towards us, there was mud on his blessed face.