Sahih Bukhari Volume 2, Book 19, Hadith 1159

فَكَانَ عَبْدُ اللَّهِ رضى الله عنه يُصَلِّي مِنَ اللَّيْلِ‏ وَكَانُوا لاَ يَزَالُونَ يَقُصُّونَ عَلَى النَّبِيِّ صلى الله عليه وسلم الرُّؤْيَا أَنَّهَا فِي اللَّيْلَةِ السَّابِعَةِ مِنَ الْعَشْرِ الأَوَاخِرِ، فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏"‏ أَرَى رُؤْيَاكُمْ قَدْ تَوَاطَتْ فِي الْعَشْرِ الأَوَاخِرِ، فَمَنْ كَانَ مُتَحَرِّيْهَا فَلْيَتَحَرَّهَا مِنَ الْعَشْرِ الأَوَاخِرِ ‏"‏‏

So after that day 'Abdullah (bin 'Umar) started offering Tahajjud. The companions of the Prophet (p.b.u.h) used to tell him their dreams that (Laila-tul-Qadr) was on the 27th of the month of Ramadan. The Prophet said, "I see that your dreams agree on the last ten nights of Ramadan and so whoever is in search of it should seek it in the last ten nights of Ramadan."