Deuteronomy 17:6

6 On the testimony of two or three witnesses a person is to be put to death, but no one is to be put to death on the testimony of only one witness.

Read Deuteronomy 17:6 Using Other Translations

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.
On 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.
But never put a person to death on the testimony of only one witness. There must always be two or three witnesses.