Hadith 500670
Chapter 2: Purity - كتاب الطهارة
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, may Allah bless him and grant him peace, 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."
Reference Muwatta Malik 500670
USC-MSA web (English) reference : Book 2, Hadith 37