Musnad Ahmad 928:6

'Affan narrated to us, Hammad bin Salamah narrated to us, 'Asim bin Bahdalah informed us, from Zirr bin Hubaysh, who said: I said to Ubayy bin Ka'b: "Indeed, Ibn Mas'ud used not to write the two *Mu'awwidhatayn* (the two surahs of seeking refuge) in his *Mushaf* (copy of the Quran)." He said: "I bear witness that the Messenger of Allah (ﷺ) informed me that Jibril said to him: 'Say: I seek refuge in the Lord of the daybreak,' so I said it. Then he said: 'Say: I seek refuge in the Lord of mankind,' so I said it." So we say what the Prophet (ﷺ) said.