Musnad Ahmad 26:434

Asbat narrated to us, he said: Al-Shaybani narrated to us, from Al-Musayyab ibn Rafi’, from Ibn Mas’ud, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever kills a snake, for him are seven good deeds. And whoever kills a gecko, for him is one good deed. And whoever leaves a snake out of fear of its retaliation is not one of us."