Musnad Ahmad 26:301

Aswad ibn Amir narrated to us, Sharik informed us, from Abu Ishaq, from Silah, from Abdullah, that the messenger of Musaylimah came to the Prophet, may Allah bless him and grant him peace, and said to him: "Do you bear witness that I am the Messenger of Allah?" He said something to him, and the Prophet, may Allah bless him and grant him peace, said to him: "Were it not that I do not kill messengers — or if I were to kill any of the messengers — I would have killed you."