Deuteronomy 17:6
6 On the evidence of two witnesses or three witnesses, he who is to die shall be put to death; he shall not be put to death on the evidence of one witness.
Deuteronomy 19:15
15 “A single witness shall not rise up against a man on account of any iniquity or any sin which he has committed; on the evidence of two or three witnesses a matter shall be confirmed.
Matthew 18:16
16 But if he does not listen to you, take one or two more with you, so that by the mouth of two or three witnesses every fact may be confirmed.