Sahih Bukhari Volume 8, Book 73, Hadith Number 187.

Narated By Ibn ‘Umar : The Prophet 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.”