Mishkat al-Masabih 16 Retaliation كتاب القصاص 84

وَعَنْ جَرِيرٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي حَجَّةِ الْوَدَاعِ: «لَا تَرْجِعُنَّ بَعْدِي كُفَّارًا يَضْرِبُ بَعْضُكُمْ رِقَاب بعض»

Jarir reported God’s Messenger as saying at the Farewell Pilgrimage, “You must never revert to infidelity after my death and cut off one another’s heads.” (Bukhari and Muslim.)