The Prophet ﷺ said during the Farewell Pilgrimage, "Woe to you," or he said, "Woe to you, do not revert to disbelief after me, striking the necks of one another."
حدثنا محمد بن جعفر حدثنا شعبة عن واقد بن محمد بن زيد أنه سمع أباه يحدث
عن النبي ﷺ أنه قال في حجة الوداع ويحكم أو قال ويلكم لا ترجعوا بعدي كفارا يضرب بعضكم رقاب بعض
Its isnad is sahih. Bukhari reported it in 10:458, 12:170, and 13:22,23, and Muslim in 1:33,34, through the route of Shubah from Waqid bin Muhammad. Suyuti also attributed it in Jami Saghir 9767 to Abu Dawud, Nasai, and Ibn Majah, but he failed to attribute it to Sahih Muslim.