By the mouth of witnesses - i. e. two witnesses, at the least (compare the marginal references). The provisions of this and the following verses protect the enactments of this chapter from abuse. The cities of refuge were not intended to exempt a criminal from deserved punishment.
Related Commentaries of Numbers 35:30
Numbers 35:30
30 Whoso killeth any person, the murderer shall be put to death by the mouth of witnesses: but one witness shall not testify against any person to cause him to die.