מדרש על במדבר 35:32

(Bamidbar 35:32) "And you shall not take ransom for one who fled to the city of his refuge, etc.": If one killed unwittingly, I might think that if he gave money (i.e., "ransom"), he would not be exiled. It is, therefore, written "And you shall not take ransom for one who has fled, etc."