It was narrated that Thawban said:
"The Messenger of Allah said: 'Adhere to righteousness even though you will not be able to do all acts of virtue. Know that the best of your deeds is Salat (prayer) and that no one maintains his ablution except a believer.'" (DE)
English reference (DE): Vol. 1, Book 1, Hadith 277 (DE)
Arabic reference (DE): Book 1, Hadith 290 (DE)