The Torah doesn't say explicitly that Yiscah (Gen. 11:29) is Sarah, but the Talmud (Megillah 14a) does say so. Rashi (on v.29) adds that the name "Yiscah" is related to the word "nesichah" ("anointed one", i.e., royalty), just as the name "Sarah" means "ruler" (from the root "sherarah"), so that the names are synonymous, and are appropriate names for a woman who was the first Jewish matriarch. Evidently she was much more commonly known as Sarah (or originally Sarai), so that name is used everywhere else (especially after
G-d renamed Sarai "Sarah"), but it's not unprecedented that a woman should have two names and that the names should not be used equally often; the best-known example (though from a much later period) is Hadassah/Esther. In Gen. 20:12 Abraham says that Sarah is his father's "daughter", but if we take this to mean "granddaughter", and we assume that the Haran in v.29 is the same as the Haran in vv.26-28, this is consistent with the Talmud's statement.
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