Zirr bin Hubaysh (may Allah have mercy on him) said: I went in a delegation to Sayyiduna Uthman (may Allah be pleased with him) and met Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him). I said to him: Narrate to me, for I have longed to meet you, and I have come only to meet you; so tell me about Laylat al-Qadr, for Sayyiduna Ibn Mas'ud (may Allah be pleased with him) says: Whoever stands (in night prayer) the whole year will find it - or will attain it. Sayyiduna Ubayy (may Allah be pleased with him) said: He knows that it is in Ramadan, but he wanted (its exact date) to remain vague for you, whereas it is the twenty-seventh night, on the basis of the sign that the Messenger of Allah (peace and blessings be upon him) told us of, which we remembered and came to know. the Messenger of Allah (peace and blessings be upon him) used to spend it continuously (in worship) until the pre-dawn time. A day before it and a day after it, he would climb up and look at the sun, and say: "On its morning the sun rises with no strong rays until it rises high."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9551