مُوَطَّأُ مَالِكٍ
Muwatta Malik 224
Forgetfulness in Prayer
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, (ﷺ), said, "I forget or I am made to forget so that I may establish the sunna."
Arabic/English book reference: Book 4, Hadith 2