مُوَطَّأُ مَالِكٍ
Muwatta Malik 67
Purity
وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ اسْتَقِيمُوا وَلَنْ تُحْصُوا وَاعْمَلُوا وَخَيْرُ أَعْمَالِكُمُ الصَّلاَةُ وَلاَ يُحَافِظُ عَلَى الْوُضُوءِ إِلاَّ مُؤْمِنٌ .
Yahya related to me from Malik that he had heard that the Messenger of Allah, (ﷺ), said, "Try to go straight, although you will not be able to do so. Act, and the best of your actions is the prayer. And only a mumin is constant in his wudu."
Arabic/English book reference: Book 2, Hadith 37