Bekhorot 56A

מה בניך ⁠— אין בלקוח ובמתנה, אף צאנך ובקרך ⁠— אינו בלקוח ובמתנה.

Just as the redemption of your firstborn son does not apply to a child who was purchased or received as a gift, i.e., the mitzva applies only to one’s own son, so too, the mitzva of separating tithe from your flock and your herd does not apply to animals that were purchased or received as a gift.

והאי בבכור כתיב! אמר קרא: ״כן תעשה״, אם אינו ענין לבכור, דלא איתיה בעשייה, דברחם קדוש ⁠— תנהו ענין למעשר בהמה.

The Talmud raises a difficulty: But this verse, which juxtaposes the firstborn son to an animal, is written with regard to redemption of a firstborn, not with regard to animal tithe. The Talmud explains that the verse states: “So you should do with your oxen, and with your sheep: 7 days it should be with its mother; on the 8th day you should give it to Me” (Exodus 22:29). If this is not needed for the matter of firstborn animals, which do not require any action to sanctify them as the animal is already consecrated when it leaves the womb, apply it to the matter of animal tithe, where an animal is sanctified only when the owner counts it as the tenth animal.

ואימא: תנהו ענין לחטאת ולאשם! דומיא דבנך ⁠— מה בנך שאינו בא על חטא, אף צאנך ושורך שאינו בא על חטא.

The Talmud suggests: But instead one can say to apply this phrase: “So you should do,” to the matter of a sin offering and a guilt offering rather than animal tithe, i.e., one should derive that these offerings are sanctified only if they originally belonged to the individual who is attempting to sanctify them, not if they are purchased or received as a gift. The Talmud explains that one expounds the phrase as referring to animal tithe because that case is similar to the case of: Your son. Just as your son does not come to be redeemed to atone for a sin, so too, the phrase: Your sheep and your oxen, discussed in the verse, is referring to animal tithe, which does not come to the Temple to atone for a sin. Sin offerings and guilt offerings are brought only if one commits a sin.