From Sahih Bukhari (Hadith 6166):
Narrated Ibn `Umar:The Prophet (ﷺ) said, "Wailakum" (woe to you) or "waihakum" (May Allah be merciful to you)." Shu`ba is not sure as to which was the right word. "Do not become disbelievers after me by cutting the necks of one another
Reference: Bukhari 6166