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