For the source text click/tap here: Kiddushin 27
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From Steinsaltz:
The Mishna (26a) teaches that when the beit din rules that an individual is obligated to take an oath on a specific matter, other issues that are in dispute can also be included in the oath, even if they are matters that ordinarily would not require an oath.
The Gemara on our daf discusses this rule, called gilgul shevua – literally, “rolling over”, or extension, of an oath. Ulla teaches that the source for the concept of gilgul shevua is the law of sota. We find that a sota who is required to take an oath that she did not commit adultery listens to the words of the kohen who is officiating at the ceremony and responds “Amen, Amen.” This response is interpreted by the Gemara to include not only her denial of an adulterous relationship with this specific man, but also another man and not only at this time, but at different points in her life, as well.
We explore the curious derivation of gilgul shavua from of all places the Sotah ritual.