Numbers 35:30
Whoever kills 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.
Nave's Topical Index—AKJV

Deuteronomy 17:6
At the mouth of two witnesses, or three witnesses, shall he that is worthy of death be put to death; but at the mouth of one witness he shall not be put to death.
Nave's Topical Index—AKJV

Deuteronomy 19:15
One witness shall not rise up against a man for any iniquity, or for any sin, in any sin that he sins: at the mouth of two witnesses, or at the mouth of three witnesses, shall the matter be established.
Nave's Topical Index—AKJV