It is narrated from Sayyiduna Abu Sa'id (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) used to observe i'tikaf; then he mentioned the full hadith. In it is that he (peace and blessings be upon him) said: "I was shown this night, then I was made to forget it. However, I saw myself prostrating in the morning in mud (water and clay)." Sayyiduna Abu Sa'id (may Allah be pleased with him) says: With my own eyes I saw the traces of mud on the forehead and nose of the Messenger of Allah (peace and blessings be upon him) on the morning of the twenty-first.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2651