Sahih Muslim Book 26, Hadith Number 5553.

Chapter : Killing of snakes.

‘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.