Musnad Ahmad 4037

It was narrated that Ibn Mas'ood said: We were with The Messenger of Allah (ﷺ) in a cave and (the soorah) "By the winds (or angels or the Messengers of Allah)" (al-Mursalat 77:1) was revealed to him; we learned it from him. Then a snake came from the side of the cave, and he said: "Kill it,” so we rushed to kill it but it got away from us. He said: “It was protected from your evil as you were protected from its evil."