Bekhorot 57A

ואזדא רבי יוחנן לטעמיה, דאמר רבי אסי אמר רבי יוחנן: האחין שחלקו ⁠— לקוחות הן, ומחזירין זה לזה ביובל.

And R' Yoḥanan follows his established line of reasoning in this regard. As Rav Asi says that R' Yoḥanan says: Brothers who divided real estate received as an inheritance are considered purchasers who have purchased from each other, and as purchasers of land they must return the portions to each other in the Jubilee Year, at which point they may redistribute the property. This demonstrates that R' Yoḥanan does not hold that it is retroactively established that each brother’s portion was designated for him upon their father’s death. Rather, it is considered as though all the land was joint property until the brothers traded or bought their respective portions from each other.

וצריכי, דאי אשמעינן הכא, בהא קאמר רבי יוחנן דומיא דבנך, מה בנך בברור לך, אף שורך וצאנך בברור לך; אבל שדה מכר הוא דאמר רחמנא ליהדר ביובל, ירושה ומתנה לא!

The Talmud notes: And both statements of R' Yoḥanan are necessary. As, if he had taught this halakha to us only here, with regard to animal tithe, one might have thought that it is in this case that R' Yoḥanan says each one has not received his own portion. This is because the case must be similar to the case of your firstborn son, in accordance with the juxtaposition between the two cases. Just as your firstborn son is redeemed only if it is clear to you that he is your son, not if he is purchased from another, so too, your cattle and your sheep are subject to animal tithe only if it is clear to you, i.e., if there is no aspect of acquisition at all. But in the case of a field, one might think it is only with regard to a field that is sold that God states that the purchaser should return it in the Jubilee Year, whereas a field that is an inheritance or a gift need not be returned.