“Whosoever saves a single life, saves an entire universe.” (From the Mishnah, Sanhedrin 4:5.) The entire surrounding passage is noteworthy:
How are witnesses inspired with awe in capital cases? They are brought in and admonished as follows: In case you may want to offer testimony that is only conjecture or hearsay or secondhand evidence, even from a person you consider trustworthy; or in the event you do not know that we shall test you by cross-examination and inquiry, then know that capital cases are not like monetary cases. In monetary cases, a man can make monetary restitution and be forgiven, but in capital cases both the blood of the man put to death and the blood of his [potential] descendants are on the witness’s head until the end of time. For thus we find in the case of Cain, who killed his brother, that it is written: ‘The bloods of your brother cry unto Me’ (Genesis 4:10) — that is, his blood and the blood of his potential descendants…. Therefore was the first man, Adam, created alone, to teach us that whoever destroys a single life, the Bible considers it as if he destroyed an entire world. And whoever saves a single life, the Bible considers it as if he saved an entire world. Furthermore, only one man, Adam, was created for the sake of peace among men, so that no one should say to his fellow, ‘My father was greater than yours…. Also, man [was created singly] to show the greatness of the Holy One, Blessed be He, for if a man strikes many coins from one mold, they all resemble one another, but the King of Kings, the Holy One, Blessed be He, made each man in the image of Adam, and yet not one of them resembles his fellow. Therefore every single person is obligated to say, ‘The world was created for my sake.’
And a second translation for comparison:
How were the witnesses awestruck in criminal cases? They were brought in and warned: Perhaps your testimony is based only on a supposition, or on hearsay, or on that of another witness, or you have had it from a trustworthy man; or perhaps you are not aware that finally we will investigate the matter by examination and cross-examination. You may also be aware of the fact that there is no similarity between civil and criminal cases. In civil cases one may repay the money damage and he is atoned; but in criminal cases the blood of the person executed, and of his descendants to the end of all generations, clings to the originator of his execution. So do we find in the case of Cain, who slew his brother. It reads [Gen. iv. 10]: “The voice of the ‘bloods’ of thy brother are crying unto me from the ground.” It does not read “blood,” but “bloods,” which means his blood and the blood of his descendants. Therefore the man was created singly, to teach that he who destroys one soul of a human being, the Scripture considers him as if he should destroy a whole world, and him who saves one soul of Israel, the Scripture considers him as if he should save a whole world. And also because of peace among creatures, so that one should not say: My grandfather was greater than yours; and also that the heretic shall not say: There are many creators in heaven; and also to proclaim the glory of the Holy One, blessed be He. For a human being stamps many coins with one stamp, and all of them are alike; but the King of the kings of kings, the Holy One, blessed be He, has stamped every man with the stamp of Adam the First, and nevertheless not one of them is like the other. Therefore every man may say: The world was created for my sake.