Abu Saeed Khudri reported,

The Messenger of Allah ﷺ performed itikaf during the first ten days of Ramadan, then he performed itikaf during the middle ten days in a small Turkish tent with a mat hanging over its entrance.He took the mat in his hand and moved it to one side of the tent. Then he put his head out and spoke to the people, and they drew close to him. He said, "I performed itikaf during the first ten days seeking this night. Then I performed itikaf during the middle ten days. Then someone came to me and I was told that it is in the last ten days. So whoever among you wishes to perform itikaf, let him do so."So the people performed itikaf with him.He said, "I was shown that it is on an odd-numbered night, and that I would prostrate on its morning in mud and water."On the morning of the twenty-first night, he had stood in prayer until dawn, and the sky had rained, causing the mosque to leak. He saw the mud and water. When he finished the dawn prayer, he came out with mud and water on his forehead and the tip of his nose. It turned out to be the twenty-first night of the last ten days.

عن أبي سعيد الخدري، قال

إن رسول الله ﷺ اعتكف العشر الأول من رمضان، ثم اعتكف العشر الأوسط في قبة تركية على سدتها حصير قال: فأخذ الحصير بيده فنحاها في ناحية القبة. ثم أطلع رأسه فكلم الناس فدنوا منه، فقال: إني أعتكف العشر الأول، ألتمس هذه الليلة. ثم اعتكفت العشر الأوسط، ثم أتيت فقيل لي: إنها في العشر الأواخر. فمن أحب منكم أن يعتكف فليعتكف. فاعتكف الناس معه. قال: وإني أريتها ليلة وتر، وأني أسجد صبيحتها في طين وماء فأصبح من ليلة إحدى وعشرين، وقد قام إلى الصبح، فمطرت السماء. فوكف المسجد، فأبصرت الطين والماء. فخرج حين فرغ من صلاة الصبح، وجبينه وروثة أنفه فيهما الطين والماء. وإذا هى ليلة إحدى وعشرين من العشر الأواخر.