Abu Dawud
Jihad
Book Number: 008 / Hadith Number: 2755
Narated By Nu'aym ibn Mas'ud : I heard the Apostle of Allah (pbuh) 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.