Musnad Ahmad 26:785

'Affan narrated to us, Hammad ibn Salamah narrated to us, 'Asim ibn Bahdalah informed us, from Zirr ibn Hubaysh, from Ibn Mas'ud, who said: We were with the Messenger of Allah, peace and blessings of Allah be upon him, at the foot of a mountain, and he was standing praying, while they were asleep. He said: Then a snake passed by him, so we woke up, and he was saying: "The One who protected you from it protected it from you." And there was revealed to him: {By those sent forth in succession, and those that blow violently} [Al-Mursalat: 2]. And he took it while it was moist in his mouth, or his mouth was moist from it.