Naradasmriti m2.39

m2.39

sākṣikadūṣaṇe kāryaṃ pūrvasākṣiviśodhanam /
śuddheṣu sākṣiṣu tataḥ paścāt sākṣyaṃ viśodhayet //

Where the deposition of the witnesses has been objected to, it becomes necessary first of all to clear the witnesses from suspicion. When the witnesses themselves have been cleared from suspicion, he may undertake to remove the doubts which have been raised against their deposition.