Musnad Ahmad 928:10

Mus'ab ibn Salam narrated to us, Al-Ajlah narrated to us, from Al-Sha'bi, from Zirr ibn Hubaysh, from Ubayy ibn Ka'b, who said: The companions of the Messenger of Allah (peace and blessings of Allah be upon him) discussed the Night of Decree (Laylat al-Qadr). Ubayy said: "I, by the One besides whom there is no god, know which night it is. It is the night that the Messenger of Allah (peace and blessings of Allah be upon him) informed us about: the night of the twenty-seventh that passes in Ramadan. The sign of that is that the sun rises on the morning following that night appearing pale, having no rays." Salamah ibn Kuhayl claimed that Zirr informed him: "That he watched for it for three years, from the first day of entering Ramadan until its end, and he saw it rise on the morning of the twenty-seventh, appearing pale, having no rays."