It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that regarding Laylat al-Qadr, the Noble Prophet (peace and blessings be upon him) said: "Whoever wishes to seek it, let him seek it on the twenty-seventh night."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 6474