Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) led us in the Fajr prayer and left out a verse. Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) came, and he had missed part of the prayer. When the prayer was completed, he said: "O Messenger of Allah, has this verse been abrogated, or were you made to forget it?" The Prophet (peace and blessings be upon him) said: "No; rather, I was made to forget it."
Ahmad recorded it, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2358