Mutarrif (may Allah have mercy on him) narrates from Sayyiduna Mu'awiyah bin Abi Sufyan (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said concerning Laylat al-Qadr: "The twenty-seventh night."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16862