I observed i'tikaf in the first ten (nights and days) in order to seek that night (Lailat-ul-Qadr). I then observed i'tikaf in the middle ten days. Then (an angel) was sent to me and I was told that this (night) is among the last ten (nights). He who among you likes to observe i'tikaf should do so; and the people observed it along with him, and he (the Holy Prophet) said: That (Lailat-ul-Qadr) was shown to me on an odd (night) and I (saw in the dream) that I was prostrating in the morning in clay and water. So in the morning of the twenty-first night when he (the Holy Prophet) got up for dawn (prayer). there was a rainfall and the mosque dripped, and I saw clay and water. When he came out after completing the morning prayer (I saw) that his forehead and the tip of his nose had (traces) of clay and water, and that was the twenty-first night among the last ten (nights). (Using translation from Muslim 1167c)  

إِنَّ رَسُولَ اللهِ ﷺ اعْتَكَفَ الْعَشْرَ الْأُوَلَ مِنْ رَمَضَانَ ثُمَّ اعْتَكَفَ الْعَشْرَ الْأوْسَطَ فِي قُبَّةٍ تُرْكِيَّةٍ عَلَى سُدَّتِهَا قِطْعَةُ حَصِيرٍ قَالَ فَأَخَذَ الْحَصِيرَ بِيَدِهِ فَنَحَّاهَا فِي نَاحِيَةِ الْقُبَّةِ ثُمَّ أَطْلَعَ رَأْسَهُ فَكَلَّمَ النَّاسَ فَدَنَوْا مِنْهُ فَقَالَ إِنِّي أَعْتَكِفُ الْعَشْرَ الْأُوَلَ أَلْتَمِسُ هَذِهِ اللَّيْلَةَ ثُمَّ اعْتَكَفْتُ الْعَشْرَ الْأوْسَطَ ثُمَّ أَتَيْتُ فَقِيلَ لِي إِنَّهَا فِي الْعَشْرِ الْأَوَاخِرِ فَمَنْ أَحَبَّ مِنْكُمْ أَنْ يَعْتَكِفَ فَلْيَعْتَكِفْ فَاعْتَكَفَ النَّاسُ مَعَهُ قَالَ «وَإِنِّي أُرِيتُهَا لَيْلَةَ وِتْرٍ وَإِنِّي ثُمَّ ذَكَرَ كَلِمَةً مَعْنَاهَا أَسْجُدُ صَبِيحَتَهَا فِي طِينٍ وَمَاءٍ» فَأَصْبَحَ مِنْ إِحْدَى وعِشْرِينَ وَقَدْ قَامَ إِلَى الصُّبْحِ وَمَطَرَتِ السَّمَاءُ فَوَكَفَ الْمَسْجِدُ فَأَبْصَرْتُ الطِّينَ وَالْمَاءَ فَخَرَجَ حِينَ فَرَغَ مِنْ صَلَاةِ الصُّبْحِ وَجَبِينُهُ وَرَوْثَةُ أَنْفِهِ فِيهِمَا الطِّينُ وَالْمَاءُ وَإِذَا هِيَ لَيْلَةُ إِحْدَى وَعِشْرِينَ مِنَ الْعَشْرِ الْأَوَاخِرِ