Reference for Bava Kamma 180:19
קתני מיהת אמר לו רבי עקיבא וכי בפני ב"ד הכהו שיודעין כמה הכהו הא הכהו בפניהם עד נעשה דיין
We must therefore say that just as a fist is a definite object that was placed before the sight of witnesses [when the murder was committed] so also it is necessary that all other instruments should have been placed before the sight of the witnesses, which excludes the case where the instrument of killing disappeared from under the hand of the murderer who is thus free.' We see then that R. Akiba said to him, 'did the actual killing take place before the judges of the Court of Law that they should be expected to know how many times the murderer struck the victim …?' which would imply that if he had killed him in their presence, [they who were the] witnesses would have been able to act as judges! — He was arguing from the point of view of R. Simeon the Temanite but this was not his own opinion.