Sahih al-Bukhari 2882

It is related that 'Ali said, "We have nothing written down from the Prophet, may Allah bless him and grant him peace,but the Qur'an and this paper. The Prophet, may Allah bless him and grant him peace, said: "Madina is a sanctuary between 'A'ir to such-and-such a place. If someone commits an improper action in it or gives refuge to someone who commits improper actions, on him is the curse of Allah, the angels and all people. Neither repentance nor ransom will be accepted from him. The dhimma [covenant] of the Muslims is the same, and it is effected by the lowest of them. If someone breaks a Muslim's covenant, on him is the curse of Allah, the angels and all people. Neither repentance nor ransom will be accepted from him. If someone accepts the protectorship of a people without his masters's permission on him is the curse of Allah, the angels and all people. Neither repentance nor ransom will be accepted from him.'"