Siman 59

Our sages, of blessed memory, said that there are three laws that the sages received as a law without a reason, and this is one of them: One who marries a woman to his adult son in a house—he has acquired it. Specifically an adult, and specifically a virgin, and specifically his first wife, and specifically that he married him off first. And the explanation of the matters: A father who marries off his adult son and did not marry off a male son prior to him—and regarding a daughter there is no concern—and he married him to a virgin, and for this son it is his first marriage, and he cleared out a specific house that he does not dwell in, for he dwells in another house, and the father cleared this house of his vessels and his objects such that he left nothing there and made it a wedding house for his son—the son has acquired that house. And this is a law without a reason; nevertheless, there is a bit of a reason for this—that they estimated the mind of the father, that from the greatness of his joy he transferred the house to him, and since he cleared the house of all his vessels and also the son has no place to dwell, the son's mind relied upon it, and it is a logical deduction that he gave it to him. And this is like a law given to Moses at Sinai, that this transfer of ownership is without a deed and without a kinyan.

And specifically if he cleared the house such that there did not remain in it even a flask of oil or a shoe or any small thing, but if he left there even a small thing like an empty flask or some small thing that is worth only a perutah – the son has not acquired it, because there is not in this a clear sign that his intention was to transfer it to him. Therefore, there were some of the sages who intentionally left there some of their vessels so that it would not appear as if they were transferring the house to them [ibid.].

If the father designated for his son for a wedding house a house that has an upper story above it or a house that has an exedra before it, and all the details that have been explained are present such that the house was acquired by the son, we do not say that the upper story or the exedra is also his. Rather, he acquired only the house, and the upper story and the exedra remain with the father. And even though an exedra is necessary for the house, nevertheless, since he did not explicitly transfer it—he did not acquire it. And similarly, if there were two houses, one inside the other, and he married him in one of them—he did not acquire the second house. And even if he married him in the inner one, and consequently it is necessary to pass through the outer one—nevertheless, he acquired the inner one and did not acquire the outer one. For since this is a law without a stated reason, we have only what they explicitly said [see Rashbam there s.v. "shnei batim" who wrote "if you say house and exedra he acquired etc.", and according to the rule of the Geonim he acquired the exedra, but the Tosafot and the poskim did not explain it so; see there in Tosafot who wrote the opposite].

And likewise, if he designated household vessels for his wedding—the son has acquired them. And if he designated a house and household vessels for him, even though he did not acquire the house, such as if he left some of his own vessels there or if he had a storehouse of merchandise there, we do not say that he also did not acquire his vessels; rather, he acquired the household vessels and did not acquire the house. And these laws apply even in this time [Beit Shmuel].