Musnad Ahmad 928:9

Sufyan ibn 'Uyaynah narrated to us, from 'Abdah and 'Asim, from Zirr, who said: I said to Ubayy: "Your brother [Ibn Mas'ud] is erasing them from the Mushaf." It was said to Sufyan: "Ibn Mas'ud?" He did not deny it. He [Ubayy] said: "I asked the Messenger of Allah, peace and blessings of Allah be upon him, and he said: 'It was said to me, so I said.' So we say as the Messenger of Allah, peace and blessings of Allah be upon him, said." Sufyan said: "Erasing them" refers to the *Mu'awwidhatan* [Surah Al-Falaq and Surah An-Nas], and they are not in the Mushaf of Ibn Mas'ud. He used to see the Messenger of Allah, peace and blessings of Allah be upon him, seeking refuge for Al-Hasan and Al-Husayn with them, and he did not hear him recite them in any of his prayers. So he thought they were [merely] invocations for seeking refuge, and he persisted in his opinion, while the rest [of the Companions] verified that they are from the Qur'an, so they included them in it.