Musannaf Ibn Abi Shayba 18:1033

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Al-‘Ala’ ibn ‘Abd al-Karim narrated to us, he said: I heard Mujahid saying: "And whoever saves one, it is as if he had saved mankind entirely." He said: Whoever refrains from killing it, he has saved it.