Musnad Ahmad 3971

It was narrated that Abul Ahwas al-Jushami said: Whilst Ibn Mas'ood was delivering a khutbah one day, a snake appeared on the wall. He interrupted his speech, then he struck it with his stick until he killed it. Then he said: I heard the Messenger of Allah (ﷺ) say: “Whoever kills a snake, it is as if he killed a mushrik man whose blood it is permissible to shed."