Zirr bin Hubaysh said: I met Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) - and he was a somewhat stern man - and I said: O Abu al-Mundhir, may Allah have mercy on you, be gentle with me. And I asked him about Laylat al-Qadr, and he said: "It is in Ramadan, and it is the twenty-seventh night." I said: 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; we counted and remembered. By Allah, it is indeed that (night)" - he swore without saying 'In sha' Allah'.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9549