Musnad Abu Ya'la al-Mawsili 152:252

Abu Khaythama narrated to us, Yunus bin Muhammad narrated to us, Dawud bin Abi al-Furat narrated to us, from Muhammad bin Zayd, from Abu al-A'yan al-'Abdi, from Abu al-Ahwas al-Jushami, that he said: While Ibn Mas'ud was delivering a sermon one day, he saw a snake walking on the wall. He interrupted his sermon and struck it with a small stick 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."