It is narrated from Sayyiduna Thawban (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Be upright, though you will not be able to count (your deeds), and know that among the best of your deeds is prayer, and none preserves ablution (wudu) except a believer."
Hadith Referenceالمعجم الكبير للطبراني / باب الثاء / 1431