Salamah bin Nu'aym bin Mas'ud narrates from his father (may Allah be pleased with him) that he said: I heard the Messenger of Allah (peace and blessings be upon him) when the envoys of Musaylimah the Liar came; he (peace and blessings be upon him) said: "Do you two also say the like of what he says?" They said: "We say the same thing that he says." He (peace and blessings be upon him) said: "If it were the practice to kill envoys, I would have killed you both; but envoys are not killed."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18776
Harithah bin Mudarrib came to Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) and said: There was no enmity between me and any of the Arabs. When I passed by a mosque of Banu Hanifah, they had come to believe in Musaylimah. So Sayyiduna Abdullah (may Allah be pleased with him) summoned them and asked them to repent. Only Ibn al-Nawwahah did not repent. Sayyiduna Abdullah (may Allah be pleased with him) said to him: I heard the Messenger of Allah (peace and blessings be upon him) say: "An envoy is not to be killed"; otherwise I would have struck off your neck. But today you have not come as an envoy. Then he gave the order to Qarazah bin Ka'b (may Allah be pleased with him), and he struck off his neck in the market. Then he said: Whoever wishes to see Ibn al-Nawwahah lying slain in the market may see him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18777
Sayyiduna Abdullah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said to Ibn al-Nawwahah: "If you were not an envoy, I would have killed you."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18778