The Messenger of Allah ﷺ said, "Woe to you! Do not revert to disbelief after me, striking the necks of one another."
حدثنا عبد الرحمن بن إبراهيم حدثنا الوليد بن مسلم أخبرني عمر بن محمد عن أبيه
أن رسول الله ﷺ قال ويحكم أو ويلكم لا ترجعوا بعدي كفارا يضرب بعضكم رقاب بعض
Narrated by Bukhari in The Book of Expeditions, chapter on the Farewell Pilgrimage, no. 4403/6043, 6166, and by Muslim in The Book of Faith, the previous chapter, no. 66/120b, from the hadith of Umar bin Muhammad, with the same wording.