كتاب الفتن
Read the English translation first, with the original Arabic directly underneath.
Narrated Ibn `Umar:
I heard the Prophet (ﷺ) saying, "Do not revert to disbelief after me by striking (cutting) the necks of one another."
لاَ تَرْجِعُوا بَعْدِي كُفَّارًا، يَضْرِبُ بَعْضُكُمْ رِقَابَ بَعْضٍ