حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا عَبْدُ الأَعْلَى، أَخْبَرَنَا سَعِيدٌ، عَنْ أَبِي نَضْرَةَ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ 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 ‏.‏ قَالَ أَبُو دَاوُدَ لاَ أَدْرِي أَخَفِيَ عَلَىَّ مِنْهُ شَىْءٌ أَمْ لاَ ‏.‏

Narrated AbuSa'id al-Khudri: The Prophet (PBUH) 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. Translation Not Available

Book Ref: Sunan Abi Dawud Book 6 Hadith 1383
Web Ref:  Sunan Abi Dawud Book 6 Hadith 1378