Numbers 35:30
30 “If anyone kills a person, the murderer shall be put to death ion the evidence of witnesses. But no person shall be put to death on the testimony of one witness.
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 g“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. Only on the evidence of two witnesses or of three witnesses shall a charge be established.