Chapter 5, Verse 108

But if it be discovered that the two witnesses are guilty of sin, then two others shall take their place from among those against whom the former two witnesses — who were in a better position to give true evidence — sinfully deposed, and the two latter witnesses shall swear by Allah, saying, ‘Surely, our testimony is truer than the testimony of the former two, and we have not been unfair in any way; for then, indeed, we should be of the unjust.’