What is the ruling if a husband calls his wife 'O adulteress' (Ya zaniyah) and she responds, 'You committed adultery with me' (Bika zanaytu)?

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Neither the husband nor the wife incurs the Hadd punishment (for defamation or adultery). The rationale is that she has admitted to the defamation (qadhf) directed at her, similar to saying 'I believe you.' However, the Hadd for Zina (adultery) is not established against her without four confessions, and the Hadd for Qadhf is not established because she did not accuse him; rather, she admitted to her own potential wrongdoing with him, which can occur without him being an adulterer (e.g., if he believed she was his wife while she knew he was a stranger). Furthermore, the statement allows for interpretation where she means to negate the accusation against both of them or that he only had relations with her, which, if not Zina, means he is complicit but not adulterous. The Hadd is waived when there is ambiguity (shubhah).