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.
Sunan Abi Dawud Book 15, Hadith 285
← Previous Hadith The Prophet (ﷺ) said: If anyone kills a man whom he grants protection prematurely, Allah will forbid him to enter Paradise.

Sunan Abi Dawud Book 15, Hadith 284 Next Hadith → Harithah ibn Mudarrib said that he came to Abdullah ibn Mas'ud and said (to him): There is no enmity between me and any of the Arabs. I passed a mosque of Banu Hanifah. They (the people) believed in Musaylimah. Abdullah (ibn Mas'ud) sent for them. They were brought, and he asked them to repent, except Ibn an-Nawwahah. He said to him: I heard the Messenger of Allah (ﷺ) say: Were it not that you were not a messenger, I would behead you. But today you are not a messenger. He then ordered Qarazah ibn Ka'b (to kill him). He beheaded him in the market. Anyone who wants to see Ibn an-Nawwahah slain in the market (he may see him).

Sunan Abi Dawud Book 15, Hadith 286