Matthew 18:16

but, if he hear you not, take with you yet one or two, that at the mouth of two witnesses or three, every word may be established.

2 Corinthians 13:1

This is the third time I am coming to you. At the mouth of two witnesses, and of three, shall every word be established.

John 8:17

And in your law also it has been written, 'The testimony of two men is true.'

1 Timothy 5:19

Against an elder receive not an accusation, except on the testimony of two or three witnesses.

Hebrews 10:28

Anyone, having set aside Moses' law, dies without mercy on the testimony of two or three witnesses;

Matthew 26:60-61

and they did not find any, though many false witnesses came forward. But afterward two false witnesses, coming forward,

Revelation 11:3-7

And I will give to My two witnesses, and they shall prophesy a thousand two hundred and sixty days, clothed in sackcloth."

Treasury of Scripture Knowledge

Summary

One witness shall not rise up against a man for any iniquity, or for any sin, in any sin that he sinneth: at the mouth of two witnesses, or at the mouth of three witnesses, shall the matter be established.

General references

Bible References

At the mouth

Matthew 18:16
but, if he hear you not, take with you yet one or two, that at the mouth of two witnesses or three, every word may be established.
Matthew 26:60
and they did not find any, though many false witnesses came forward. But afterward two false witnesses, coming forward,
John 8:17
And in your law also it has been written, 'The testimony of two men is true.'
2 Corinthians 13:1
This is the third time I am coming to you. At the mouth of two witnesses, and of three, shall every word be established.
1 Timothy 5:19
Against an elder receive not an accusation, except on the testimony of two or three witnesses.
Hebrews 10:28
Anyone, having set aside Moses' law, dies without mercy on the testimony of two or three witnesses;
Revelation 11:3
And I will give to My two witnesses, and they shall prophesy a thousand two hundred and sixty days, clothed in sackcloth."

General references

John 7:51
"Does our law judge a man, unless it first hear from him, and know what he does?"