Bava Batra 36A

״לדידי אמר לי גוי, דמינך זבנה״⁠ – מהימן. מי איכא מידי דאילו גוי אמר ⁠– לא מהימן, ואילו אמר ישראל משמיה דגוי ⁠– מהימן?!

The non-Jew told me that he purchased a field from you, this claim is deemed credible. The Talmud asks: Is there any case where if a non-Jew says it he is not deemed credible, but if a Jew said it in the non-Jew’s name he would be deemed credible?

אלא אמר רבא, אי אמר ישראל: ״קמי דידי זבנה גוי מינך, וזבנה ניהלי״⁠ – מהימן, מיגו דאי בעי אמר ליה: אנא זבינתה מינך.

Rather, Rava said: If a Jew said to the prior owner: A non-Jew purchased a field from you in my presence, and then he sold it to me, this claim is deemed credible, since if he wanted to, he could have said to the prior owner of the land: I purchased it from you.

ואמר רב יהודה: האי מאן דנקיט מגלא ותובליא, ואמר: ״איזיל איגזרה לדקלא דפלניא, דזבנתיה מיניה״⁠ – מהימן, לא חציף איניש למיגזר דקלא דלאו דיליה.

The Talmud records a series of halakhot pertaining to presumptive ownership. And Rav Yehuda says: This one who is holding a sickle and rope and says: I will go cull the dates from the date tree of so-and-so, from whom I purchased it, is deemed credible. The reason for this is that a person is not so brazen that he would cull the dates from a date tree that is not his.

ואמר רב יהודה: האי מאן דאחזיק מגודא דערודי ולבר, לא הוי חזקה. מאי טעמא? מימר אמר: כל דזרע נמי ערודי אכלי ליה.

And Rav Yehuda says: With regard to this one, who possesses a field only from the fence built to prevent the entry of the wild donkeys and outward toward the public property, this conduct is not sufficient to establish the presumption of ownership. What is the reason? The owner says to himself: Everything that he sows, the wild donkeys will eat as well, and cannot establish the presumption of ownership for him, as he is not profiting from the land as an owner would.