Musnad Abu Ya'la al-Mawsili 152:29

Ibrahim ibn al-Hajjaj narrated to us, Salam Abu al-Mundhir narrated to us, Asim narrated to us, from Abu Wa'il, from Abdullah ibn Mas'ud: That Musaylimah sent two men, one of whom was Ibn Athal ibn Hajar. The Messenger of Allah (peace be upon him) said, "Do you bear witness that Muhammad is the Messenger of Allah?" They said, "We bear witness that Musaylimah is the Messenger of Allah." The Prophet (peace be upon him) said, "I believe in Allah and His messengers. If I were one to kill envoys, I would have killed you both." While Ibn Mas'ud was in Kufa, the man who was with Ibn Athal—who was a relative of his—was brought before him, and he ordered his execution. He said to the people, "Do you know why I killed this man?" They said, "We do not know." He said, "Musaylimah sent this man with Ibn Athal ibn Hajar, and the Messenger of Allah (peace be upon him) said, 'Do you bear witness that Muhammad is the Messenger of Allah?' They said, 'We bear witness that Musaylimah is the Messenger of Allah.' The Prophet (peace be upon him) said, 'I believe in Allah and His messengers. If I were one to kill envoys, I would have killed you both.' So, for that reason, I killed him." Abu Wa'il said: And the man was, on that day, a disbeliever.