Musnad Ahmad 26:826

Ya'qub ibn Ibrahim narrated to us, my father narrated to us, from Ibn Ishaq, who said: Abd al-Rahman ibn al-Aswad ibn Yazid al-Nakha'i narrated to me, from his father, from Abdullah ibn Mas'ud, who said: "The Surah 'Al-Mursalat 'Urfa' (The Messengers sent forth) was revealed to the Messenger of Allah, peace and blessings of Allah be upon him, on the night of the snake." He said: So we said to him, "And what is the night of the snake, O Abu Abd al-Rahman?" He said: "While we were with the Messenger of Allah, peace and blessings of Allah be upon him, at Hira at night, a snake came out upon us from the mountain. The Messenger of Allah, peace and blessings of Allah be upon him, commanded us to kill it. We sought it, but it eluded us. He said: 'Leave it be, for Allah has protected it from your evil, just as He has protected you from its evil.'"