Jihad (Kitab Al-Jihad)
Dawud :: Book 14 : Hadith 2755
Narrated Nu'aym ibn Mas'ud:
I heard the Apostle of Allah (peace_be_upon_him) say when he read the letter of Musaylimah: What do you believe yourselves? They said: We believe as he believes. He said: I swear by Allah that were it not that messengers are not killed, I would cut off your heads.