Musnad Ahmad 30:2051

Mu'awiyah narrated to us, he said: Za'idah narrated to us, from al-A'mash, from Abu Salih, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him: "Whoever affiliates himself with a people without the permission of his masters, upon him is the curse of Allah, the angels, and all of mankind. Neither a ransom nor a compensatory payment will be accepted from him on the Day of Resurrection. And Medina is sacred; so whoever commits an innovation therein or harbors an innovator, upon him is the curse of Allah, the angels, and all of mankind. Allah will not accept from him a ransom nor a compensatory payment on the Day of Resurrection. And the protection (dhimmah) of the Muslims is one; the lowest of them strives to uphold it. So whoever violates the protection of a Muslim, upon him is the curse of Allah, the angels, and all of mankind. Neither a ransom nor a compensatory payment will be accepted from him on the Day of Resurrection."