Reference for Bava Kamma 180:12
ואיבעית אימא הכי קאמר ליה הא אנא דסבירא לי כרבי יוסי הגלילי דאמר מנה צורי והא סהדי דמסהדי בך זיל הב ליה מנה צורי
What is the meaning of, 'Here I am, and here is also R. Jose the Galilean'? If you say he meant, 'Here I am who witnessed you [doing this] and here is also R. Jose the Galilean who holds that the payment should be a Tyrian <i>maneh</i>; go therefore and thus pay him a Tyrian maneh', would this not imply that a witness is eligible to act [also] as judge? But [how can this be, since] it was taught: If the members of the Sanhedrin saw a man killing another, some of them should act as witnesses and the others should act as judges: this is the opinion of R. Tarfon. R. Akiba [on the other hand] said that all of them are considered witnesses and [they thus cannot act as judges, for] a witness may not act as a judge.<span class="x" onmousemove="('comment',' Sanh. 34b; B.B. 114a. ');"><sup>9</sup></span>