Translation:Shulchan Aruch/Orach Chaim/586
586 (3) Blowing a shofar which is an object of idol worship where the shofar is owned by a Jew, does not fulfill the mitzvah, as such use for idol worship is never nullified and it is as if the shofar has been reduced to a size too small to use. One should not blow a shofar owned by an idol worshipper which was an object of worship or was used in the worship of idols, but if one did blow such a shofar one has fulfilled the mitzvah. So too, if one did not intend that his action would transfer the shofar to become one’s own possession, but if one intended that the shofar would become one’s possession then one has not performed the mitzvah, because it becomes an object of idol worship owned by a Jew. [And there are those who are stringent, who regard even blowing a shofar of an idol worshipper as not a performance of the mitzvah, unless the idol worship was nullified before the holiday.]
586 (4) Blowing the shofar of an animal that was sacrificed to idol worship, even if owned by an idol worshipper, does not perform the mitzvah as such a shofar is eternally nullified.