Reference for Bava Kamma 236:1

מ"ט דרב חסדא הואיל ואנקטה נגרי ברייתא

The reason of R. Hisda is because [living things] have the habit of running out<span class="x" onmousemove="('comment',' So that where the proprietor did not know of the theft he should be notified about the restoration so as to take more care of his sheep. ');">¹</span> into the fields.<span class="x" onmousemove="('comment',' Cf. supra 57a. ');">²</span>

Jastrow