Hadith 502240
Chapter 4: Forgetfulness in Prayer - كتاب السهو
وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " إِنِّي لأَنْسَى أَوْ أُنَسَّى لأَسُنَّ " .
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."
Reference Muwatta Malik 502240
USC-MSA web (English) reference : Book 4, Hadith 2