Hazrat Adi bin Adi (may Allah have mercy on him) narrates that Hazrat Umar bin Abdul Aziz (may Allah have mercy on him) wrote to me and said: After praise and salutations, indeed faith has certain obligations, rules, limits, and regulations. So whoever fulfills them, his faith becomes complete, and whoever does not fulfill them, his faith also does not become complete. So if I remain alive, soon I will explain them to you so that you may act upon them, and if I die before telling you, then I am not very desirous of your company.