Zirr bin Hubaysh narrates from Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) that he said: "Laylat al-Qadr is the twenty-seventh night." I said: O Abu al-Mundhir, how did you come to know that? He said: "By the sign that the Messenger of Allah (peace and blessings be upon him) told us of; I remembered it and counted. By the One who sent down the Book upon Muhammad (peace and blessings be upon him), it is the twenty-seventh night."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9550