Reference for Bava Batra 5:12
ליתני האי ולא ליתני האי סיפא אצטריכא ליה ובכתבי הקדש אע"פ ששניהם רוצים לא יחלוקו):
How then have you explained the Mishnah? As applying to a courtyard in which there is no right of division. If it is speaking of one in which there is no right of division, even if both owners consent, what does it matter? Either of them can retract? — R. Assi answered in the name of R. Johanan: We assume that each made a formal contract with the other, by means of a <i>kinyan</i>.<span class="x" onmousemove="('comment',' Lit., 'acquisition': the handing of a small article, usually a piece of cloth, by one of the contracting parties to the other, as a symbol that the object transferred has passed from the ownership of the one to that of the other. v. B.M. 47a. ');"><sup>10</sup></span>