Shevuos, Chapter 4, Mishnah 12. Our Mishnah deals with the principle that the demand upon potential witnesses to come and testify must come from the mouth of the claimant himself - the person making the monetary claim.
The words of the Mishnah:
"Shilach b'yad avdo" - the claimant does not administer the oath to the witnesses himself, but rather sends his slave in his place to make the demand of them.
"O she'amar lo hanitva: mashbia ani aleichem she'im atem yod'in lo eidus shetavo'u useta'idu" - although this is an unusual scenario, the case is one in which it is specifically the defendant, the person against whom the claim is being made, who administers the oath to the witnesses that they come and testify on behalf of his opponent who is claiming from him (and what motivated him to do so, we do not know).
"Harei elu peturin ad sheyishme'u mipi hatove'a" - in both cases the witnesses are exempt, since the claim must be heard from the mouth of the person making the monetary claim: not from the mouth of the opposing party, and not even from the mouth of his slave.
Why did the Mishnah specify "b'yad avdo"?
The commentators find the wording of the Mishnah difficult: why does it not simply say that he sent by means of some other agent? What is unique specifically about his slave and servant? The commentators differ in explaining this:
Some understand that another person, even though he is not the claimant and is not making the monetary claim himself, can come with authorization - with a power of attorney - and represent the claimant's interests. One who comes with authorization can be the one who administers the oath, and he has the power to obligate the potential witnesses in an oath or in testimony. A slave, however, cannot come with authorization, and therefore in all scenarios he has no power to obligate an oath or testimony.
Others understand that a slave too can come with authorization and power of attorney, and our Mishnah is dealing with one who comes without authorization. Another agent does not represent the interest and does not obligate an oath and testimony unless he has authorization, whereas the slave - "the hand of a slave is like the hand of his master" - represents his master's affairs even without authorization. For this reason the Mishnah specified "b'yad avdo": even though he does not require authorization, the witnesses are not obligated until they hear it from the mouth of the claimant himself.
Running after the witnesses:
Following on from this, the Gemara says: if the witnesses saw the claimant running after them, and they sensed or knew that he intended to administer an oath to them that they come and testify on his behalf, and they preempted him and said to him, "Why are you running after us? We swear that we know no testimony for you" - they are exempt. Even though they sense or suspect that he is about to make a claim and administer an oath to them, they did not actually hear it from him, and since they did not hear a claim from his mouth, this is not "from the mouth of the claimant," and therefore there is no obligation of an oath or of testimony.
In summary: our Mishnah establishes that the obligation of the oath of testimony applies only when the claim is heard from the mouth of the claimant himself. Therefore the witnesses are exempt when the claimant sends his slave in his place, and likewise when it is the defendant who administers the oath. We examined the dispute among the commentators as to why the Mishnah specified "his slave" - whether because a slave cannot come with authorization at all, or because he does not require authorization, based on the principle that "the hand of a slave is like the hand of his master" - and the statement of the Gemara that even one who senses that an oath is about to be administered to him is exempt as long as he has not actually heard the claim from the mouth of the claimant.