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
← Previous Hadith Sulaym ibn Amir, a man of Himyar, said: There was a covenant between Mu'awiyah and the Byzantines, and he was going towards their country, and when the covenant came to an end, he attacked them. A man came on a horse, or a packhorse saying, Allah is Most Great, Allah is Most Great; let there be faithfulness and not treachery. And when they looked they found that he was Amr ibn Abasah. Mu'awiyah sent for him and questioned him (about that). He said: I heard the Messenger of Allah (ﷺ) say: When one has covenant with people he must not strengthen or loosen it till its term comes to an end or he brings it to an end in agreement with them (to make both the parties equal). So Mu'awiyah returned.

Sunan Abi Dawud Book 15, Hadith 283 Next Hadith → 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