Punishment: Death Penalty: Not Inflicted by the Testimony of Less than Two Witnesses
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Numbers 35:30
If anyone kills a person, the murderer is to be put to death on the testimony of the witnesses. But no one is to be put to death based on the testimony of a lone witness.
Deuteronomy 17:6
On the testimony of two or three witnesses a man shall be put to death, but he shall not be executed on the testimony of a lone witness.
Deuteronomy 19:15
A lone witness is not sufficient to establish any wrongdoing or sin against a man, regardless of what offense he may have committed. A matter must be established by the testimony of two or three witnesses.