Musnad Ahmad 928:18

'Affan narrated to us, Hammad bin Zayd narrated to us, 'Asim narrated to us, from Zirr, who said: I said to Ubayy bin Ka'b: "O Abu al-Mundhir, inform me about the Night of Decree (Laylat al-Qadr)." He mentioned the hadith. He said: So I said: "O Abu al-Mundhir, how did you know that?" He said: "By the sign which the Messenger of Allah, may Allah bless him and grant him peace, informed us of."