Musnad Abu Ya'la al-Mawsili 152:179

Abu Khaythama narrated to us, Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from Asim, from Abu Wa'il, from Abdullah, that the Prophet (may Allah bless him and grant him peace) said to a man: "Were it not that you are a messenger, I would have killed you." Meaning the messenger of Musaylima.