(a) 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 for ten days in the middle ten days of Ramadan. One year he performed i'tikaf until, when it was the twenty-first night, which is the night at the end of which he would come out of i'tikaf, he said: "Those who have performed i'tikaf with me should also perform i'tikaf in the last ten days. I was shown this night (Laylat al-Qadr), then I was made to forget it, and I saw myself prostrating in water and mud the following morning. So seek it in the odd nights of the last ten."
(b) Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) says: It rained that night, and the roof of the mosque was made of twigs and branches, so it leaked. Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) says: My eyes saw marks of mud on the blessed forehead and nose of the Messenger of Allah (peace and blessings be upon him), and that was the morning of the twenty-first.
(c) Imam al-Bukhari (may Allah have mercy on him) says: Al-Humaydi used to cite this hadith as evidence that a person should not wipe his forehead during prayer, because the mark was seen on the pure forehead and nose of the Prophet (peace and blessings be upon him) after he had finished praying.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3551