Mishkat al-Masabih 3 Purification كتاب الطهارة 11

عَنْ ثَوْبَانَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «اسْتَقِيمُوا وَلَنْ تُحْصُوا وَاعْلَمُوا أَنَّ خَيْرَ أَعْمَالِكُمُ الصَّلَاةُ وَلَا يُحَافِظُ عَلَى الْوُضُوءِ إِلَّا مُؤْمِنٌ» . رَوَاهُ مَالِكٌ وَأَحْمَدُ وَابْنُ مَاجَهْ وَالدَّارِمِيُّ

Thauban reported God’s messenger as saying, “Keep to the straight path, which you will not be able to do, know that your best action is prayer and that only a believer observes ablution carefully.” Malik, Ahmad, Ibn Majah and Darimi transmitted it.