Sunan Abu Dawud
2761
Sahih+3

Narrated Nu'aym ibn Mas'ud: I heard the Messenger of Allah (ﷺ) say when they (messengers sent by Musaylimah) 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

SahihAl-Albani
SahihMuhammad Muhyi Al-Din Abdul Hamid