From the Mishnah, Tractate Sanhedrin Chapter 4
Therefore, humans were created singly, to teach you that whoever destroys a single soul [of Israel], Scripture accounts it as if he had destroyed a full world; and whoever saves one soul of Israel, Scripture accounts it as if he had saved a full world. And for the sake of peace among people, that one should not say to his fellow, "My father greater than yours;" and that heretics should not say, "There are many powers in Heaven." Again, to declare the greatness of the Holy One, blessed be God, for one stamps out many coins with one die, and they are all alike, but the King, the King of kings, the Holy One, blessed be God, stamped each person with the seal of Adam, and not one of them is like his fellow. Therefore each and every one is obliged to say, "For my sake the world was created."