Musnad Ahmad 928:29

'Abdullah narrated to us, Ahmad ibn Muhammad ibn Ayyub narrated to us, Abu Bakr ibn 'Ayyash narrated to us, from 'Asim, from Zirr, who said: I came to Medina and entered the mosque, and there I was with Ubayy ibn Ka'b. I came to him and said: "May Allah have mercy on you, Abu al-Mundhir, lower your wing for me," for he was a man in whom there was some harshness. I asked him about the Night of Decree (Laylat al-Qadr), and he said: "The night of the twenty-seventh." I said: "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, so we counted and memorized. And the sign of it is that the sun rises on the morning following it like a washbasin, having no rays, until it rises high."