Qays bin Abi Hazim (may Allah have mercy on him) said: A man came to Sayyiduna Ibn Mas'ud (may Allah be pleased with him) and said: I passed by one of the mosques of Banu Hanifah and heard them reciting something that Allah did not reveal: "By those who grind, grinding; by those who knead, kneading; by those who bake, baking; and by those who feed morsels, feeding." (The narrator) said: Then Sayyiduna Ibn Mas'ud (may Allah be pleased with him) brought forward their imam, Ibn al-Nawwahah, and had him executed, and he left the rest (considering how many they were) and said: "I will not slaughter them for Satan today. Send them away to Syria, until Allah grants them repentance or the plague wipes them out."
(The narrator) said: Isma'il informed me from Qays bin Abi Hazim that Sayyiduna Ibn Mas'ud (may Allah be pleased with him) said: This one, meaning Ibn al-Nawwahah, came to the Messenger of Allah (peace and blessings be upon him); Musaylimah had sent him to him (peace and blessings be upon him). The Prophet (peace and blessings be upon him) said: "If I were one to kill an envoy, I would have killed you."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8924