Once, the Holy Prophet (sa) was leading the Fajr [pre-dawn] prayer, during which he forgot to recite a verse in his recitation [of the Holy Quran]. Hazrat Ubayy (ra) was not able to join the congregation from the beginning of the prayer, instead he joined in the middle [of the prayer]. Upon completing the prayer, the Holy Prophet (sa) enquired if anyone in the congregation had noticed anything different in his recitation. Everyone remained silent. Then, the Holy Prophet (sa) asked whether Ubayy (ra) bin Kaab was present. By this time, Hazrat Ubayy (ra) had completed offering his Salat. Despite joining the congregation late, he must have joined by the second rak‘ah [unit of prayer] to have been able to hear the mistake that was made or notice the verse which had been forgotten.
In any case, Hazrat Ubayy (ra) bin Kaab had completed offering his Salat and informed the Holy Prophet (sa) that he had not recited such and such verse. He said, “O Messenger (sa) of Allah, you did not recite such and such verse during your recitation; has it been abrogated or did you simply forget to recite it?” The Holy Prophet (sa) replied that he had merely forgotten to recite it. Then, the Holy Prophet (sa) said to Ubayy (ra), “I knew you would be the only one to notice this.”
(Sheikh Shah Moinuddin Ahmad Nadvi, Siyar al-Sahabah, Vol. 3 [Karachi, Pakistan: Dar al-Ishaah Urdu Bazar, 2004] 148)