Numbers 35:32 "And you shall take no ransom from one who must flee to the city of refuge"

A person who commits a murder unintentionally is instructed to flee to a special "refuge city".

He is obligated to live there and may not leave until the High Priest dies (see Positive Mitzvah 182).

Many people would be willing to pay large sums of money rather than be forced to remain in exile in a refuge city for so many years. We are forbidden to accept this money as a ransom.