At the beginning of his Caliphate:

Allah has sent down a guiding Book wherein He has explained virtue and vice. You should adopt the course of virtue whereby you will have guidance, and keep aloof from the direction of vice so that you remain on the right way. Mind the obligations. Fulfil them for Allah and they will take you to Paradise. Allah has made unlawful the things which are not unknown and made lawful the things which are without defect. He has declared paying regard to Muslims as the highest of all regards. He has placed the rights of Muslims in the same grade as devotion (to Himself and His oneness). Therefore, a Muslim is one from whose tongue and hand every Muslim is safe save in the matter of truth. It is not, therefore, lawful to molest a Muslim except when it is obligatory.

Hasten towards the most common matter which is peculiar to every one; and that is death. Certainly, people (who have gone) are ahead of you while the hour (Judgement Day ) is driving you from behind. Remain light, in order that you may overtake them. Your backs are being awaited for the sake of the fronts. Fear Allah in the matter of His creatures and His cities because you will be questioned even about lands and beasts. Obey Allah and do not disobey Him. When you see virtue adopt it, and when you see vice avoid it.