Musnad Ahmad 38:14

Yahya ibn Sa'id narrated to us, from Sufyan, Salama ibn Kuhayl narrated to us, from Dharr, from Sa'id ibn 'Abd al-Rahman ibn Abza, from his father: That the Prophet, peace and blessings of Allah be upon him, prayed the Fajr prayer and omitted an ayah. When he finished praying, he said, "Is Ubayy ibn Ka'b among the people?" Ubayy said, "O Messenger of Allah, was such-and-such an ayah abrogated, or did you forget it?" He said, "I forgot it."