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
Abu Salamah bin Abd al-Rahman (may Allah have mercy on him) narrates: We were discussing Laylat al-Qadr among a group of Quraysh, so I got up and went to Abu Sa'id al-Khudri (may Allah be pleased with him) and said: O Abu Sa'id! Will you not come out with us to the palm grove? He said: Yes. He called for a striped woollen cloak and wrapped it around himself, and we went out. I said: Did you hear the Messenger of Allah (peace and blessings be upon him) mention Laylat al-Qadr? He said: Yes. We performed i'tikaf with the Messenger of Allah (peace and blessings be upon him) in the middle ten days of Ramadan. On the morning of the twentieth of Ramadan he stood among us and said: "Whoever has gone out, let him return, for I was shown Laylat al-Qadr and then forgot it, so seek it in the odd nights of the last ten. And I was shown that I was prostrating in water and mud." We could not see a single wisp of cloud in the sky. Then the iqamah for the prayer was called, a cloud rose up, and rain fell on us until the roof of the mosque streamed, and their roof in those days was made of palm branches. I saw the Messenger of Allah (peace and blessings be upon him) prostrating in mud and water, until I saw the trace of mud on the tip of his nose and on his forehead.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8591
Sayyidah Aishah (may Allah be pleased with her), the wife of the Prophet (peace and blessings be upon him), narrates: I would not enter (the house) except for a need, and if I came across a sick person while passing, I would ask after him. The Messenger of Allah (peace and blessings be upon him) would put his blessed head out towards me while he (peace and blessings be upon him) was in the mosque, and I would comb it; and he (peace and blessings be upon him) would not enter the house except for a need when he (peace and blessings be upon him) was in i'tikaf.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8592
Sayyidah Aishah (may Allah be pleased with her) narrates that the Prophet (peace and blessings be upon him) used to perform i'tikaf in the last ten days of Ramadan until Allah took him in death; then his wives performed i'tikaf after him (peace and blessings be upon him).
The sunnah of i'tikaf is that the mu'takif does not go out except for an unavoidable need, does not visit the sick, does not touch a woman nor have intimate relations, and does not perform i'tikaf except in a congregational mosque; and it is also for the mu'takif that he fasts.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8593