Musnad Ahmed: Hadith 3685

When Ibn an-Nawwahah was killed: This man and Ibn Uthal came to the Prophet (ﷺ) as envoys of Musailimah the Liar. The Messenger of Allah (ﷺ) said to them. "Do you bear witness that I am the Messenger of Allah?" They said: We bear witness that Musailimah is the messenger of Allah! He said: "If I were to kill any envoy, I would have struck your necks." And it became a precedent that envoys were not to be killed. As for Ibn Uthal. Allah, may He be glorified and exalted, took care of him for us. And as for this one, he persisted in his misguidance until Allah enabled the Muslims to capture him now.