Reference for Chullin 78:9

אי נמי ע"כ לא קאמר ר"א התם אלא גבי עובד כוכבים דסתם מחשבת עובד כוכבים לעבודת כוכבים אבל ישראל הוכיח סופו על תחלתו לא אמרינן

perhaps the Rabbis maintain their view only there [in our Mishnah] because we did not hear him [sc. the idolater] express any intention at all, but here since we heard him express an intention [after the slaughtering, even the Rabbis will admit that it is invalid, for] his last act proves what he had in mind at the beginning.<span class="x" onmousemove="('comment',' That the slaughtering was, without doubt, intended for idolatry.');">⁵</span>

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