Related על סוטה 59:5

Rabbi Eliezer says: Hallah may be taken from [dough] that is clean, [in order to exempt] that which is unclean. How [may this be done]? [If one has] clean dough and unclean dough, he takes sufficient hallah out of the [clean] dough whose hallah has not yet been taken, and puts [dough] less than the size of an egg in the middle, in order that he may take off [the hallah] from what is close together. But the sages prohibit.