הלכה על שבועות 39:23

And you may ask, "How is it that the one who makes a vow not to eat something that he is commanded about to eat, does not eat it - as behold, he is commanded about that thing with a positive commandment; and a positive commandment comes and pushes off the negative commandment of 'he shall not break.' As so did the Sages say in every place, 'a positive commandments comes and pushes off a negative commandment.'" The answer to you is that the vow is a positive commandment and there is [also] a negative commandment in it: the negative commandment is "he shall not break," and the positive commandment is "everything that comes out of his mouth he shall do." And from the reason that we gave [before] that his body is acted upon with an oath, they said (Shevuot 20a) [that] one that is added on with an oath [of someone else] is exempt [from it]; but with vows, he is obligated. How so? If he heard that his fellow made a vow and he said, "I am also like you," within the time of speech (immediately after it), behold this one is forbidden, since the intention of this one is to say, [just] like you are forbidden from this thing, so too will I be forbidden from it, and with this, it will be sufficient for him. But with an oath, wherein we picture the first one as if he acts upon his body with his words - as we have said - he has not removed something else away from his body; they, may their memory be blessed, did not see that this later one is included in this movement by saying, "I am also like you." [Rather,] he must speak out the expression of this movement with his actual mouth - for example, if he says, "I too swear like you"; or if he hears from the mouth of someone who moves him [specifically] to that thing, and he fulfills it and indicates that he wants this movement, like when another man says to him, "I put an oath upon you," and he answers, "Amen."