From Sahih Bukhari (Hadith 121):
Narrated Jarir:The Prophet (ﷺ) said to me during Hajjat-al-Wida`: Let the people keep quiet and listen. Then he said (addressing the people), "Do not (become infidels) revert to disbelief after me by striking the necks (cutting the throats) of one another (killing each other)
Reference: Bukhari 121