Sunan Ibn Majah
277
Sahih+3

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

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi