ENOfficial
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.
Source: sunnah.com
Other translations
Arabic
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
sunnah.com