Musnad Ahmad 26:199

Abdullah ibn Yazid and Yunus narrated to us, they said: Dawud—meaning Ibn Abi al-Furat—narrated to us, from Muhammad ibn Zayd, from Abu al-A'yan al-'Abdi, from Abu al-Ahwas al-Jushami, who said: While Ibn Mas'ud was delivering a sermon one day, he noticed a snake crawling on the wall. He interrupted his sermon, then struck it with his rod—Yunus said: with his rod—until he killed it. Then he said: I heard the Messenger of Allah, may Allah bless him and grant him peace, say: "Whoever kills a snake, it is as if he has killed a polytheist man whose blood was lawful."