It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that a man saw in a dream that Laylat al-Qadr is on the twenty-seventh night of Ramadan. So the Noble Prophet (peace and blessings be upon him) said: "I see that your dreams coincide and agree upon the last seven nights, so whoever among you seeks Laylat al-Qadr should seek it in the odd nights of the last ten nights."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4547