Tosefta על עבודה זרה 83:20

A Jew may not tie up his livestock beside the livestock of a Gentile (i.e., on the Gentile's property), for if his livestock [went there] to eat on its own [see Ab. Zar. 42a:9 (מאליה)], [the Gentile] is not required to return it. [Presumably, livestock that becomes accustomed to being brought onto someone else's property will eventually wander there on its own.]