Sahih Muslim
The Book on Salutations and Greetings (Kitab As-Salam)
Book Number: 026 / Hadith Number: 5553
'Abdullah reported: We were with Allah's Messenger (may peace be upon him) in a cave when there was revealed to him (the Sura al-Mursalat, i. e. Sura lxxvii.:" By those sent forth to spread goodness" ) and we had just heard (it) from his lips that there appeared before us a snake. He said: Kill it. We hastened to kill it, but it slipped away from us, thereupon Allah's Messenger (may peace be upon him) said: Allah saved it from your harm just as he saved you from its evil.