Whoever destroys a soul, it is considered as if he destroyed an entire world. And whoever saves a life, it is considered as if he saved an entire world.

The Talmud (AD 200-500) Collection of Jewish rabbinical writings

Mishnah Sanhedrin 4:9; Yerushalmi Talmud, Tractate Sanhedrin 37a
    (Source)

Alt. trans.: "Whoever destroys a single life is as guilty as though he had destroyed the entire world; and whoever rescues a single life earns as much merit as though he had rescued the entire world."