Al-Sunabihi narrates from Sayyiduna Bilal (may Allah be pleased with him), and he from the Prophet (peace and blessings be upon him), that he (peace and blessings be upon him) said: "Laylat al-Qadr (the Night of Decree) is the twenty-fourth night."
Hadith Referenceالمعجم الكبير للطبراني / باب الباء / 1098