Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrated: We were with the Messenger of Allah (peace and blessings be upon him) in a cave, and Surah "Wal-Mursalati 'urfa" was revealed to him (peace and blessings be upon him). I learned it from his mouth while his mouth was still moist from reciting it. I do not remember with which of the two the recitation was ended: 'Then in what statement after it will they believe?' [Surah al-Mursalat: 50] or 'And when it is said to them, "Bow," they do not bow' [Surah al-Mursalat: 48]. He (the narrator) said: During that, a snake slipped into a hole and escaped, so he (peace and blessings be upon him) said: "You have been protected from its harm, and it has been protected from your harm."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10118