Bava Batra 53A

שלא בפניו ⁠– צריך למימר ליה: ״לך חזק וקני״.

But if the act was performed not in the seller’s presence, the seller must say to him: Go, take possession and thereby acquire the property for him to acquire it.

בעי רב: מתנה ⁠– היאך? אמר שמואל: מאי תבעי ליה לאבא? השתא ומה מכר, דקא יהיב ליה זוזי, אי אמר ליה: ״לך חזק וקני״⁠ – אין, אי לא ⁠– לא; מתנה, לא כל שכן?! ורב סבר: מאן דיהיב מתנה, בעין יפה יהיב.

Rav raises a dilemma: How does one acquire a gift, i.e., is it necessary for the giver to say: Go, take possession and thereby acquire? Shmuel said: What dilemma is raised to Abba, i.e., Rav? Now one could say the following: And what is the halakha with regard to a sale, where the buyer is giving money to the seller? If the seller says to the buyer: Go, take possession and thereby acquire the property, the acquisition does take effect, but if he did not say this, it does not. Therefore, with regard to a gift, where no money is given to the seller, is it not all the more so reasonable that the acquisition not take effect without a clear directive from the seller? The Talmud answers: And Rav holds that it is possible to say that one who gives a gift gives it generously, and would allow the acquisition even absent a clear directive.

וכמה כל שהוא? כדשמואל ⁠– דאמר שמואל: גדר גדר והשלימו לעשרה, ופרץ פרצה כדי שיכנס ויצא בה ⁠– הרי זו חזקה.

§ The Mishnah teaches that taking possession can be performed by building a fence or breaching a fence even a bit. The Talmud clarifies: And how much is the measure of a bit? It is in accordance with the statement of Shmuel, as Shmuel says: If one had previously built a fence, and now completed it to a height of 10 handbreadths, which is the height of a halakhically significant barrier; or similarly, if one had previously breached a breach, and now expanded it in order that it be large enough that a person can enter and exit through it, this is considered taking possession.