Mishkat al-Masabih #3821

Ibn Mas'ud told that when Ibn an-Nawwaha and Ibn Uthal, Musailima’s messengers, came to the Prophet he asked them whether they testified that he was God’s Messenger. On their replying, "We testify that Musailima is God’s messenger,” he said, "I believe in God and His Messenger. If it was my custom to kill a messenger I would kill you.” ‘Abdallah (i.e. Ibn Mas'ud) said that the sunna that a messenger should not be killed then came into force. Ahmad transmitted it.