Reference for Bava Kamma 67:13
<big><strong>מתני׳</strong></big> שור שוה מאתים שנגח שור שוה מאתים ואין הנבילה יפה כלום אמר ר"מ על זה נאמר (שמות כא, לה) ומכרו את השור החי וחצו את כספו
If the improvement was due to fattening, how could you explain the opening clause, 'where … the injured ox [subsequently] improved and reached the value of four hundred <i>zuz</i> … compensation will be paid as at the time of the damage'? For where the improvement was due to the act of fattening [by the owner], what need could there have been to state [that compensation for the original damage has still to be paid]? — R. Papa thereupon said: The ruling in the opening clause<span class="x" onmousemove="('comment',' Dealing with the case where it was the injured ox that improved and increased in value. ');"><sup>12</sup></span>