“You may not stand by the blood of your friend!” (Vayikra 19:16)
It is a negative commandment that one must not hold oneself back from saving a life of a fellow Jew when one sees him in danger of death, and one has the power to save him, as it says “You may not stand by the blood of your friend!”. Chazal said, “How do we know that one who sees his friend drowning in a river, being dragged by a wild animal or being attacked by armed robbers, is obligated to save him? The answer is that we see this obligation in the verse You may not stand by the blood of your friend!”. One is obligated not only to save him oneself, but if one cannot save him oneself, one must even hire other people to save him. Included in this prohibition is the law that one must not hold oneself back from testifying on one’s friend’s behalf, when by so doing one will cause him loss of money.
The root of this commandment is well known, for in the same way that he saves his friend, so too his friend will save him and this will help towards the inhabitation of the world. Hashem Himself desires the inhabitation of the world, as He created it to be inhabited.
This commandment applies in all places and at all times, to men and women. One who transgresses it and withholds himself from saving another’s life when he had the ability to do so, has transgressed this prohibition.