The Messenger of Allah ﷺ used to spend the middle ten days of Ramadan in retirement and devotion (i'tikaf) in the mosque. One year he had retirement and devotion in the mosque (as usual); when the twenty-first night came, and this night when he used to come out his devotion in the mosque, he said: He who has engaged himself in devotion along with me should do so during the last ten days; I saw that night, that was caused to forget it, but I have seen myself prostrating in water and mud on the morning following (that night), so seek it in the last ten days and seek it every night with an odd number. Abu sa'id said: Rain fell that night, the mosque that was thatched building dripped, and my eyes saw the Messenger of Allah ﷺ with the traces of water and mud, on his forehead on the morning following the twenty-first night.  

كَانَ رَسُولُ اللَّهِ ﷺ يَعْتَكِفُ الْعَشْرَ الأَوْسَطَ مِنْ رَمَضَانَ فَاعْتَكَفَ عَامًا حَتَّى إِذَا كَانَتْ لَيْلَةُ إِحْدَى وَعِشْرِينَ وَهِيَ اللَّيْلَةُ الَّتِي يَخْرُجُ فِيهَا مِنَ اعْتِكَافِهِ قَالَ مَنْ كَانَ اعْتَكَفَ مَعِي فَلْيَعْتَكِفِ الْعَشْرَ الأَوَاخِرَ وَقَدْ رَأَيْتُ هَذِهِ اللَّيْلَةَ ثُمَّ أُنْسِيتُهَا وَقَدْ رَأَيْتُنِي أَسْجُدُ مِنْ صَبِيحَتِهَا فِي مَاءٍ وَطِينٍ فَالْتَمِسُوهَا فِي الْعَشْرِ الأَوَاخِرِ وَالْتَمِسُوهَا فِي كُلِّ وِتْرٍ قَالَ أَبُو سَعِيدٍ فَمُطِرَتِ السَّمَاءُ مِنْ تِلْكَ اللَّيْلَةِ وَكَانَ الْمَسْجِدُ عَلَى عَرِيشٍ فَوَكَفَ الْمَسْجِدُ فَقَالَ أَبُو سَعِيدٍ فَأَبْصَرَتْ عَيْنَاىَ رَسُولَ اللَّهِ ﷺ وَعَلَى جَبْهَتِهِ وَأَنْفِهِ أَثَرُ الْمَاءِ وَالطِّينِ مِنْ صَبِيحَةِ إِحْدَى وَعِشْرِينَ  

The Prophet ﷺ said: Seek it (laylat al-Qadr) in the last ten days of Ramadan. Seek it on the ninth, seventh and fifth night. I (AbuNadrah) said: You know counting better than us, AbuSa'id. He said: Yes. I asked: What do you mean by the ninth, seventh and fifth night? He said: When the twenty-first night passes, the night which follows it is the night; when the twenty-third night passes, the night which follows it is the seventh; when the twenty-fifth passes, the night which follows it is the fifth. Abu Dawud said: I do not know whether anything remained hidden from me or not.  

قَالَ رَسُولُ اللَّهِ ﷺ الْتَمِسُوهَا فِي الْعَشْرِ الأَوَاخِرِ مِنْ رَمَضَانَ وَالْتَمِسُوهَا فِي التَّاسِعَةِ وَالسَّابِعَةِ وَالْخَامِسَةِ قَالَ قُلْتُ يَا أَبَا سَعِيدٍ إِنَّكُمْ أَعْلَمُ بِالْعَدَدِ مِنَّا قَالَ أَجَلْ قُلْتُ مَا التَّاسِعَةُ وَالسَّابِعَةُ وَالْخَامِسَةُ قَالَ إِذَا مَضَتْ وَاحِدَةٌ وَعِشْرُونَ فَالَّتِي تَلِيهَا التَّاسِعَةُ وَإِذَا مَضَى ثَلاَثٌ وَعِشْرُونَ فَالَّتِي تَلِيهَا السَّابِعَةُ وَإِذَا مَضَى خَمْسٌ وَعِشْرُونَ فَالَّتِي تَلِيهَا الْخَامِسَةُ