Some men amongst the companions of the Prophet ﷺ were shown in their dreams that the night of Qadr was in the last seven nights of Ramadan. Messenger of Allah ﷺ said, "It seems that all your dreams agree that (the Night of Qadr) is in the last seven nights, and whoever wants to search for it (i.e. the Night of Qadr) should search in the last seven (nights of Ramadan).  

أَنَّ رِجَالاً مِنْ أَصْحَابِ النَّبِيِّ ﷺ أُرُوا لَيْلَةَ الْقَدْرِ فِي الْمَنَامِ فِي السَّبْعِ الأَوَاخِرِ فَقَالَ رَسُولُ اللَّهِ ﷺ أَرَى رُؤْيَاكُمْ قَدْ تَوَاطَأَتْ فِي السَّبْعِ الأَوَاخِرِ فَمَنْ كَانَ مُتَحَرِّيَهَا فَلْيَتَحَرَّهَا فِي السَّبْعِ الأَوَاخِرِ  

I asked Abu Saʿid, and he was a friend of mine, (about the Night of Qadr) and he said, "We practiced Iʿtikaf (seclusion in the mosque) in the middle third of the month of Ramadan with the Prophet ﷺ . In the morning of the 20th of Ramadan, the Prophet ﷺ came and addressed us and said, 'I was informed of (the date of the Night of Qadr) but I was caused to forget it; so search for it in the odd nights of the last ten nights of the month of Ramadan. (In the dream) I saw myself prostrating in mud and water (as a sign). So, whoever was in Iʿtikaf with me should return to it with me (for another 10-day's period)', and we returned. At that time there was no sign of clouds in the sky but suddenly a cloud came and it rained till rainwater started leaking through the roof of the mosque which was made of date-palm leaf stalks. Then the prayer was established and I saw Messenger of Allah ﷺ prostrating in mud and water and I saw the traces of mud on his forehead."  

سَأَلْتُ أَبَا سَعِيدٍ وَكَانَ لِي صَدِيقًا فَقَالَ اعْتَكَفْنَا مَعَ النَّبِيِّ ﷺ الْعَشْرَ الأَوْسَطَ مِنْ رَمَضَانَ فَخَرَجَ صَبِيحَةَ عِشْرِينَ فَخَطَبَنَا وَقَالَ إِنِّي أُرِيتُ لَيْلَةَ الْقَدْرِ ثُمَّ أُنْسِيتُهَا أَوْ نُسِّيتُهَا فَالْتَمِسُوهَا فِي الْعَشْرِ الأَوَاخِرِ فِي الْوَتْرِ وَإِنِّي رَأَيْتُ أَنِّي أَسْجُدُ فِي مَاءٍ وَطِينٍ فَمَنْ كَانَ اعْتَكَفَ مَعَ رَسُولِ اللَّهِ ﷺ فَلْيَرْجِعْ فَرَجَعْنَا وَمَا نَرَى فِي السَّمَاءِ قَزَعَةً فَجَاءَتْ سَحَابَةٌ فَمَطَرَتْ حَتَّى سَالَ سَقْفُ الْمَسْجِدِ وَكَانَ مِنْ جَرِيدِ النَّخْلِ وَأُقِيمَتِ الصَّلاَةُ فَرَأَيْتُ رَسُولَ اللَّهِ ﷺ يَسْجُدُ فِي الْمَاءِ وَالطِّينِ حَتَّى رَأَيْتُ أَثَرَ الطِّينِ فِي جَبْهَتِهِ  

Messenger of Allah ﷺ used to practice Iʿtikaf (in the mosque) in the middle third of Ramadan and after passing the twenty nights he used to go back to his house on the 21st, and the people who were in Iʿtikaf with him also used to go back to their houses. Once in Ramadan, in which he practiced Iʿtikaf, he established the night prayers at the night in which he used to return home, and then he addressed the people and ordered them whatever Allah wished him to order and said, "I used to practice Iʿtikaf for these ten days (i.e. the middle third but now I intend to stay in Iʿtikaf for the last ten days (of the month); so whoever was in Iʿtikaf with me should stay at his place of seclusion. I have verily been shown (the date of) this Night (of Qadr) but I have forgotten it. So search for it in the odd nights of the last ten days (of this month). I also saw myself (in the dream) prostrating in mud and water." On the night of the 21st, the sky was covered with clouds and it rained, and the rainwater started leaking through the roof of the mosque at the praying place of the Prophet ﷺ . I saw with my own eyes the Prophet at the completion of the morning prayer leaving with his face covered with mud and water.  

عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ ؓ كَانَ رَسُولُ اللَّهِ ﷺ يُجَاوِرُ فِي رَمَضَانَ الْعَشْرَ الَّتِي فِي وَسَطِ الشَّهْرِ فَإِذَا كَانَ حِينَ يُمْسِي مِنْ عِشْرِينَ لَيْلَةً تَمْضِي وَيَسْتَقْبِلُ إِحْدَى وَعِشْرِينَ رَجَعَ إِلَى مَسْكَنِهِ وَرَجَعَ مَنْ كَانَ يُجَاوِرُ مَعَهُ وَأَنَّهُ أَقَامَ فِي شَهْرٍ جَاوَرَ فِيهِ اللَّيْلَةَ الَّتِي كَانَ يَرْجِعُ فِيهَا فَخَطَبَ النَّاسَ فَأَمَرَهُمْ مَا شَاءَ اللَّهُ ثُمَّ قَالَ كُنْتُ أُجَاوِرُ هَذِهِ الْعَشْرَ ثُمَّ قَدْ بَدَا لِي أَنْ أُجَاوِرَ هَذِهِ الْعَشْرَ الأَوَاخِرَ فَمَنْ كَانَ اعْتَكَفَ مَعِي فَلْيَثْبُتْ فِي مُعْتَكَفِهِ وَقَدْ أُرِيتُ هَذِهِ اللَّيْلَةَ ثُمَّ أُنْسِيتُهَا فَابْتَغُوهَا فِي الْعَشْرِ الأَوَاخِرِ وَابْتَغُوهَا فِي كُلِّ وِتْرٍ وَقَدْ رَأَيْتُنِي أَسْجُدُ فِي مَاءٍ وَطِينٍ فَاسْتَهَلَّتِ السَّمَاءُ فِي تِلْكَ اللَّيْلَةِ فَأَمْطَرَتْ فَوَكَفَ الْمَسْجِدُ فِي مُصَلَّى النَّبِيِّ ﷺ لَيْلَةَ إِحْدَى وَعِشْرِينَ فَبَصُرَتْ عَيْنِي رَسُولَ اللَّهِ ﷺ وَنَظَرْتُ إِلَيْهِ انْصَرَفَ مِنَ الصُّبْحِ وَوَجْهُهُ مُمْتَلِئٌ طِينًا وَمَاءً  

Messenger of Allah ﷺ used to practice Iʿtikaf in the last ten nights of Ramadan and used to say, "Look for the Night of Qadr in the last ten nights of the month of Ramadan."  

كَانَ رَسُولُ اللَّهِ ﷺ يُجَاوِرُ فِي الْعَشْرِ الأَوَاخِرِ مِنْ رَمَضَانَ وَيَقُولُ تَحَرَّوْا لَيْلَةَ الْقَدْرِ فِي الْعَشْرِ الأَوَاخِرِ مِنْ رَمَضَانَ