Bava Metzia, Chapter 1, Mishnah 3. At the conclusion of the previous Mishnah, we discussed the rule "one who lifts a lost object for his friend - his friend acquires it": when a person lifts an ownerless object from the ground on behalf of his friend, his friend acquires it. Our Mishnah comes to discuss the parameters of this law, in a case where two people are before us: one is riding on an animal, and the other is walking beside him on the ground.
The text of the Mishnah:
"Hayah rochev al gabei behemah vera'ah es hametziah, ve'amar lachaveiro: tenah li" - The rider sees an object resting on the ground that he wishes to acquire for himself. This involves an object that does not carry an obligation of returning a lost item - a beautiful, sparkling stone or something similar - and he turns to the one walking on the ground and asks him to give it to him.
The precision in the wording "tenah li":
This wording, although it seems technical and semantic, is precise: it does not contain an instruction to acquire the found object on behalf of the rider, but rather a request to give it into his hands. This means that the actual act of acquisition does not take effect until the object reaches the hands of the rider.
The first case: "Netalah ve'amar: ani zachisi bah - zachah bah":
If the one walking on the ground lifted the stone and said "I acquired it" - meaning, I lifted it and it is mine, and from the moment of lifting I acquired it - he is the owner. Even though he lifted it following the rider's words, the rider never commanded him to acquire it on his behalf. We do not know what was in the lifter's heart, and there is no basis to assume that he specifically intended to acquire it for the rider when he was not commanded to do so. Therefore, he is believed to say that he acquired it for himself.
However, if he had said "acquire it for me":
This case is not explicitly stated in the Mishnah: if the rider had explicitly said "acquire it for me", and the one walking on the ground had lifted it, we would assume that he acquired it for his friend - unless he opened his mouth and declared before the lifting: "I am not acquiring it for you, I am taking it for myself." And if the matter were to come before a court, they would tell him: it stands to reason that when you lifted the object you did so on the strength of the instruction to acquire it for your friend, and there is a presumption that you indeed acquired it for him. And if he claims that this was not what he thought in his heart, they will answer him: words in the heart are not words. We only know the revealed facts in reality, and the fact is that you were commanded to acquire it for him, and that is indeed what you appeared to do.
The second case: "Im mishenesano lo amar: ani zachisi bah techilah - lo amar klum":
Here the instruction was "give it to me", and the one walking on the ground indeed gave the found object into the hands of the rider. After the stone is already resting in the rider's hand, the walker says to him: I did indeed give it into your hand, but know that before I gave it I intended to acquire it for myself, and I am only handing it to you so that you may look at it, or merely as a deposit and for safekeeping.
Regarding this the Mishnah rules "lo amar klum" - his statement has no halachic validity. For by giving the object into the hands of the rider he acted according to his instruction, and there is no acquisition for himself in this act. And if he indeed thought so in his heart, we do not know this; all that is visible to our eyes is that he gave the object to the rider, and the rider took it into his hand. Therefore, it is ruled that the found object belongs to the rider.
In summary: Our Mishnah teaches that everything depends on the wording of the request and the actual deed. By saying "Tnah li" - give it to me - he did not instruct him to acquire it on his behalf, and therefore the person who picked it up and said "Ani zachiti bah" - I acquired it - acquires it; however, once he gave the found item into the hands of the rider, his claim of "Ani zachiti bah techilah" - I acquired it first - is completely meaningless, since unspoken intentions are of no consequence, and we only follow the overt action before us.