Musnad Ahmad 922:5

Abdullah narrated to us, Yahya ibn Dawud al-Wasiti narrated to us, Ishaq ibn Yusuf al-Azraq narrated to us, from Sufyan, from Salamah ibn Kuhayl, from Dharr, from Sa’id ibn Abd al-Rahman ibn Abza, from his father, from Ubayy ibn Ka’b, who said: The Prophet (peace and blessings of Allah be upon him) led us in the Fajr prayer and omitted a verse. Ubayy came, having missed part of the prayer, and when [the Prophet] finished, he said: "O Messenger of Allah, was this verse abrogated, or have you been made to forget it?" He said: "No, rather I have been made to forget it."