Bava Batra 114A

אבל בלילה, אפילו שלשה ⁠– כותבין ואין עושין דין. מאי טעמא? דהוו להו עדים, ואין עד נעשה דיין. אמר ליה: אין, הכי נמי קאמינא.

but if they came at night, even if 3 men came to visit the sick person, they may write the will and sign it as witnesses but they may not act in judgment. What is the reason that they may not act in judgment the next day? It is because they are already witnesses to the will of the deceased, and there is a principle that a witness cannot become a judge, i.e., one who acts as a witness in a particular matter cannot become a judge with regard to that same matter? Rabba bar Ḥanina said to Abaye: Yes, it is indeed so; this is what I was saying.

איתמר: קנין, עד אימתי חוזר? רבה אמר: כל זמן שיושבין. רב יוסף אמר: כל זמן שעוסקין באותו ענין.

§ After mentioning Rav Yehuda’s ruling the Talmud cites a related dispute. It was stated: With regard to a transaction, until when may one of the parties renege on the transaction? Rabba says: As long as they are seated in the same location they may renege on the transaction. Rav Yosef says: As long as they are dealing with that matter, i.e., they are still discussing that transaction, they may renege on it.

אמר רב יוסף: כוותי דידי מסתברא, דאמר רב יהודה: שלשה שנכנסו לבקר את החולה ⁠– רצו כותבין, רצו עושין דין. ואי סלקא דעתך כל זמן שיושבין, ליחוש דלמא הדר ביה!

Rav Yosef said: It stands to reason in accordance with my opinion, as Rav Yehuda says: In a case where there were 3 men who entered a room to visit a sick person and the sick person desires to write a will in order to distribute his property following his death, if the visitors wish to do so they may write his will and sign it as witnesses. And if they wish, they may act in judgment. And if it enters your mind that one may renege on a deal as long as they are seated in the same location, then let us be concerned that perhaps he will renege on his decision. How can the visitors act in judgment in a case where the matter has not been resolved, as the sick person may still change his mind?