Is customary menstrual practice established by a single instance?

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The established position (madhhab) dictates that customary menstrual practice (al-'adah) is not established by a single instance. This position relies on the derivation that custom requires repetition or recurrence (mu'awadah). The Prophet Muhammad's instruction to the woman who consulted him through Umm Salamah, where he told her to revert to the cycle preceding the condition that afflicted her, supports this view, as the term 'was' (kana) implies continuity and repetition of action, which is not achieved by a single occurrence. Furthermore, the command to leave prayer for the 'days of her quru'' (plural) indicates more than one instance, and the minimum recognized plural for 'quru'' is three.