It is narrated in marfu' form from Sayyiduna Abu Umamah al-Bahili (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Remain upright — and how excellent it is if you remain upright — and none keeps up ablution except a believer."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 8093