Allah and His Messenger—that is, anything not commanded as either obligatory or recommended.

As for anything commanded as obligatory or recommended, where that command is established by evidence from the Sharia, it belongs to the religion Allah has prescribed, even if those in authority disagree over some of these matters. This applies whether or not it was practised during the lifetime of the Prophet, may Allah bless him and grant him peace. Whatever was done after his death at his command—such as fighting the apostates, the renegade Kharijites, the Persians, the Byzantines and the Turks, expelling the Jews and Christians from the Arabian Peninsula, and other such acts—is part of his Sunnah.

For this reason, ‘Umar ibn ‘Abd al-‘Azīz used to say:

“The Messenger of Allah, may Allah bless him and grant him peace, established practices whose observance affirms the Book of Allah, brings obedience to Allah to completion, and strengthens adherence to Allah’s religion. No one may change them or entertain the opinion of anyone who opposes them. Whoever follows their guidance is rightly guided; whoever seeks victory through them is granted victory. Whoever opposes them and follows a path other than that of the believers, Allah will leave him to what he has chosen and cause him to enter Hell—an evil destination.”

The Sunnah of his rightly guided caliphs is therefore among the things commanded by Allah and His Messenger. There is detailed evidence from the Sharia for this, which is not appropriate to discuss here.

Thus, just as Allah has explained in His Book how to address the People of the Book and establish the proof against them through the signs He has made clear of Muhammad’s messengership, may Allah bless him and grant him peace; through what their own scriptures contain concerning this and what they have distorted and altered in their religion; and through confirmation of what the messengers before him brought—so that, when a learned and fair-minded person from the People of the Book hears this, he finds it all among the clearest arguments and soundest proofs.