Narrated Abu Said Al-Khudri:

Allah's Apostle used to practice Itikaf in the middle ten days of Ramadan and once he stayed in Itikaf till the night of the twenty-first and it was the night in the morning of which he used to come out of his Itikaf. The Prophet ﷺ said, "Whoever was in Itikaf with me should stay in Itikaf for the last ten days, for I was informed (of the date) of the Night (of Qadr) but I have been caused to forget it. (In the dream) I saw myself prostrating in mud and water in the morning of that night. So, look for it in the last ten nights and in the odd ones of them." It rained that night and the roof of the mosque dribbled as it was made of leaf stalks of date-palms. I saw with my own eyes the mark of mud and water on the forehead of the Prophet (i.e. in the morning of the twenty-first).

حَدَّثَنَا إِسْمَاعِيلُ، قَالَ حَدَّثَنِي مَالِكٌ، عَنْ يَزِيدَ بْنِ عَبْدِ اللَّهِ بْنِ الْهَادِ، عَنْ مُحَمَّدِ بْنِ إِبْرَاهِيمَ بْنِ الْحَارِثِ التَّيْمِيِّ، عَنْ أَبِي سَلَمَةَ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ ـ رضى الله عنه ـ أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ يَعْتَكِفُ فِي الْعَشْرِ الأَوْسَطِ مِنْ رَمَضَانَ، فَاعْتَكَفَ عَامًا حَتَّى إِذَا كَانَ لَيْلَةَ إِحْدَى وَعِشْرِينَ، وَهِيَ اللَّيْلَةُ الَّتِي يَخْرُجُ مِنْ صَبِيحَتِهَا مِنِ اعْتِكَافِهِ قَالَ
" مَنْ كَانَ اعْتَكَفَ مَعِي فَلْيَعْتَكِفِ الْعَشْرَ الأَوَاخِرَ، وَقَدْ أُرِيتُ هَذِهِ اللَّيْلَةَ ثُمَّ أُنْسِيتُهَا، وَقَدْ رَأَيْتُنِي أَسْجُدُ فِي مَاءٍ وَطِينٍ مِنْ صَبِيحَتِهَا، فَالْتَمِسُوهَا فِي الْعَشْرِ الأَوَاخِرِ، وَالْتَمِسُوهَا فِي كُلِّ وِتْرٍ ". فَمَطَرَتِ السَّمَاءُ تِلْكَ اللَّيْلَةَ، وَكَانَ الْمَسْجِدُ عَلَى عَرِيشٍ فَوَكَفَ الْمَسْجِدُ، فَبَصُرَتْ عَيْنَاىَ رَسُولَ اللَّهِ ﷺ عَلَى جَبْهَتِهِ أَثَرُ الْمَاءِ وَالطِّينِ، مِنْ صُبْحِ إِحْدَى وَعِشْرِينَ.