Musnad Ahmad 26:161

Yazid narrated to us, Mas'udi informed us, 'Asim narrated to me, from Abu Wa'il, who said: 'Abdullah said, when Ibn al-Nawwah was killed: "This man and Ibn Athal had come to the Prophet, may Allah bless him and grant him peace, as messengers for Musaylimah the Liar. The Messenger of Allah, may Allah bless him and grant him peace, said to them: 'Do you bear witness that I am the Messenger of Allah?' They said: 'We bear witness that Musaylimah is the Messenger of Allah.' He said: 'If I were one to kill a messenger, I would have struck off your necks.' He said: So the Sunnah was established that a messenger is not to be killed. As for Ibn Athal, Allah the Almighty and Majestic sufficed us against him. As for this one, that [status] remained with him, until Allah gave power over him just now."