It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Noble Prophet (peace and blessings be upon him) said: "Whoever wants to seek Laylat al-Qadr should seek it on the twenty-seventh night," and he said, "Seek Laylat al-Qadr on the twenty-seventh night."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4808