Deuteronomy 17:6
6 qOn the evidence of two witnesses or of three witnesses the one who is to die shall be put to death; a person shall not be put to death on the evidence of one witness.
Deuteronomy 19:15
Laws Concerning Witnesses
15 “A single witness shall not suffice against a person for any crime or for any wrong in connection with any offense that he has committed. bOnly on the evidence of two witnesses or of three witnesses shall a charge be established.