Talmud for Ketubot 2:2
וּמוֹדֶה רַבִּי יְהוֹשֻׁעַ בְּאוֹמֵר לַחֲבֵרוֹ שָׂדֶה זוֹ שֶׁל אָבִיךָ הָיְתָה וּלְקַחְתִּיהָ הֵימֶנּוּ, שֶׁהוּא נֶאֱמָן, שֶׁהַפֶּה שֶׁאָסַר הוּא הַפֶּה שֶׁהִתִּיר. וְאִם יֵשׁ עֵדִים שֶׁהִיא שֶׁל אָבִיו וְהוּא אוֹמֵר לְקַחְתִּיהָ הֵימֶנּוּ, אֵינוֹ נֶאֱמָן:
And R. Yehoshua concedes that if one says to his neighbor: This field was your father's and I bought it from him, he is believed, for "the mouth that forbids is the mouth that permits." [Even though above, in the first chapter, re "If she said: 'After you betrothed me, I was forced,'" R. Yehoshua differs from R. Gamliel, saying that the woman is not believed in this (even though) she thus forbids herself to the priesthood, when she could have said: I am a mukkath etz and have been permitted to the priesthood — this is so in an instance of "forbidden" or "permitted," i.e., to be forbidden to the priesthood or permitted to it. It is in this instance that R. Yehoshua differs from R. Gamliel, saying that she is not believed with a miggo ("I could have said, etc."); but here, where there is no question of forbidden or permitted, but (rather a question of) monetary loss, as when one says to his neighbor: "This field was your father's and I bought it from him," R. Yehoshua concedes to R. Gamliel that in such an instance he is believed with a miggo, having been able to say: "It is mine," so that if he says: "It was your father's, and I bought it from him," he is believed.] And if there are witnesses that it was his father's, and he says: I bought it from him, he is not believed. [The gemara explains that this Mishnah speaks of an instance in which he (the possessor of the field) ate from it only two years before (i.e., in the lifetime of) the father and one year before the son. The Mishnah apprises us that since the three years of chazakah (possession) were not completed in the father's lifetime, the year that he ate before the son does not accrue to the sum of the years of chazakah.