Muwatta Malik 223

وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ إِنِّي لأَنْسَى أَوْ أُنَسَّى لأَسُنَّ ‏"‏ ‏.‏

Translation

Yahya related to me from Malik that he had heard that the Messenger of Allah, may Allah bless him and grant him peace, said, "I forget or I am made to forget so that I may establish the sunna