I heard the Messenger of Allah ﷺ say to the two messengers, after they had read the letter of Musaylima the Liar, "What do the two of you say?"They replied, "We say what he says."The Messenger of Allah ﷺ said, "By Allah, were it not that messengers are not to be killed, I would have struck off both your heads."
سمعت رسول الله ﷺ يقول للرسولين حين قرآ كتاب مسيلمة الكذاب: فما تقولان أنتما؟ قالا: نقول كما قال. فقال رسول الله ﷺ: أما والله لولا أن الرسل لا تقتل لضربت أعناقكما.
It was narrated by Abu Dawud (2761), Ahmad (15989), Tirmidhi in Ilal al-Kabir (2/953), and Hakim (3/52-53, and 2/142-143), all of them through various routes from Muhammad bin Ishaq, who said, Saad bin Tariq Ashjai narrated to me, from Salama bin Nuaym bin Masud Ashjai, from his father, who related it.Its chain is hasan because of Muhammad bin Ishaq, for his hadith is hasan when he states his source explicitly. Tirmidhi said, "I asked Muhammad about this hadith, and he said, Ibn Abu Zaida has also narrated it from Saad bin Tariq, and he considered it a hasan hadith." Hakim said, "Sahih according to the criterion of Muslim."