مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3738
Musnad of Abdullah bin Mas'ood
It was narrated that 'Abdullah bin Mas'ood (رضي الله عنه) said: Ibn an-Nawwahah and Ibn Uthal cane as envoys of Musailimah to the Prophet (ﷺ) and he said to them: "Do you bear witness that I am the Messenger of Allah?" They said: We bear witness that Musailimah is the messenger of Allah! He said: "I believe in Allah and His Messengers. If I were to kill any envoy, I would have killed you.” ‘Abdullah said: And it became a precedent that envoys were not to be killed
Musnad Ahmad 3761A saheeh hadeeth; this is a da’eef isnad]