I [Rabbi Ephraim Oshry] explained to the adoptive father that his reward would be immense without recognition of having the boy called to the Torah by his name, for he had saved this child from death about which our sages say, “Whoever maintains one Jewish life is considered as if he had maintained the whole world.” G-d will doubtless reward him for his righteousness in raising the boy and educating him in the way of goodness, and grant him many blessings and bounty.
Oshry, Efroim, and B. Goldman. Responsa from the Holocaust. Rev. ed. New York: Judaica Press, 2001.
On account of this, We prescribed for the Children of Israel that whoever kills a person – unless it is for the killing a person or for creating disorder in the land – it as if he killed all mankind; and whoever saves a life, it as if he had saved the life of all mankind.
(Qur’an 5:32)
The saying actually applies to all lives, not only to Jewish lives.