Musnad Abu Ya'la al-Mawsili 152:90

Abu Khaythama narrated to us, Jarir narrated to us, from al-A'mash, from Ibrahim, from al-Aswad, who said: Abdullah ibn Mas'ud said: While we were with the Messenger of Allah (peace be upon him) in a cave, "Al-Mursalat 'urfan" (The Sent Forth by Turns) was revealed to him. We received it from his mouth, and his mouth was moist with it, when a snake emerged. The Messenger of Allah (peace be upon him) said: "Kill it." We rushed toward it, but it escaped us. The Messenger of Allah (peace be upon him) said: "It was protected from your evil, and you were protected from its evil."