Salim (may Allah have mercy on him) narrated from his father, Sayyiduna Abdullah bin Umar (may Allah be pleased with them both), that two men from the Ansar recited a surah which the Messenger of Allah (peace and blessings be upon him) had taught them, and they used to recite it. One night they stood up to pray but could not recall a single letter of it. In the morning they both went to the Messenger of Allah (peace and blessings be upon him) and mentioned this to him, and the Messenger of Allah (peace and blessings be upon him) said: "It is among what has been abrogated and caused to be forgotten, so turn your attention away from it." Al-Zuhri (may Allah have mercy on him) used to recite: "Whatever verse We abrogate or cause to be forgotten" [Surah al-Baqarah: 106] with a dammah on the nun, without doubling.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13084