Reference for Bava Kamma 86:1
מיהו בן חורין דמשלם כופר ע"פ עצמו והיכי דמי דאי אתו סהדי ואסהידו ביה דקטל ולא ידעי אי תם הוה אי מועד הוה ואמר מריה דמועד הוא דמשלם כופר על פי עצמו היכא דליכא עדים משלם דמים
But [it is only in the case of] a freeman where <i>kofer</i> may sometimes be paid on the strength of the defendant's own admission — as where witnesses appeared and testified to the ox having killed [a freeman] without, however, knowing whether it was still <i>Tam</i> or already <i>Mu'ad</i> and the owner admits it to have been <i>Mu'ad</i>, in which case <i>kofer</i> would be paid on the strength of his own admission<span class="x" onmousemove="('comment',' As the ox in this case would be subject to be stoned, [and where the ox is stoned, the owner pays kofer]. ');"><sup>1</sup></span>