Musnad Ahmad 3828

It was narrated from ‘Abdullah (رضي الله عنه) that The envoy of Musailimah came to the Prophet (ﷺ), who said to him: "Do you bear witness that I am the Messenger of Allah?” He said something to him, and the Prophet (ﷺ) said: “Were it not that I do not kill envoy - or were I to kill any envoy - I would have killed you.”