Musnad Ahmad 26:287

Sulayman ibn Dawud al-Hashimi narrated to us, Abu Bakr ibn Ayyash informed us, Asim narrated to us, from Abu Wa’il, from Ibn Mu’ayz al-Sa’di, who said: I went out to water a horse of mine at the time of the pre-dawn (sahar), and I passed by the mosque of Banu Hanifa, and they were saying: "Indeed, Musaylimah is the Messenger of Allah." So I came to Abdullah (ibn Mas’ud) and informed him. He sent the police, and they brought them. He asked them to repent, and they repented, so he let them go. But he struck the neck of Abdullah ibn al-Nawwahah. They said: "You seized a group of people for the same matter, yet you killed some of them and let others go?" He said: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, when this man and Ibn Athal ibn Hajar came to him. He said: 'Do you both testify that I am the Messenger of Allah?' They replied: 'We testify that Musaylimah is the Messenger of Allah.' The Prophet, peace and blessings of Allah be upon him, said: 'I believe in Allah and His Messengers. If I were one to kill an envoy, I would have killed you both.'" He said: "It is for that reason that I killed him."