The Prophet ﷺ said, "Do not return to disbelief after me, striking one another's necks."
حدثنا وهب بن جرير حدثنا أبي قال سمعت عبد الملك بن عمير يحدث
أن النبي ﷺ قال لا ترجعوا بعدي كفارا يضرب بعضكم رقاب بعض
Its isnad is sahih. It is found in Majma al-Zawaid 7:295, where it says, "Ahmad, Abu Yaala, Bazzar, and Tabarani reported it, and their narrators are the narrators of the sahih."