Musnad Ahmad 928:14

Yahya ibn Sa'id narrated to us, from Sufyan, 'Asim narrated to me, from Zirr, who said: I said to my father [Ubayy ibn Ka'b]: Inform me about the Night of Decree (Laylat al-Qadr), for the son of Umm 'Abd [Ibn Mas'ud] used to say: "Whoever stands in prayer for the whole year will attain it." He said: May Allah have mercy on Abu 'Abd al-Rahman. He knew that it is in Ramadan, and that it is on the twenty-seventh, but he wanted to keep the people in doubt so that they would not rely [solely on that night]. By the One who sent down the Book upon Muhammad, it is in Ramadan on the night of the twenty-seventh. I said: O Abu al-Mundhir, how did you know that? He said: By the sign which the Messenger of Allah, peace and blessings of Allah be upon him, informed us of. We counted and we memorized. By Allah, it is indeed it, without exception. I said to Zirr: What is the sign? He said: "That the sun rises on the morning following it like a basin (tast), having no rays."