What is the minimum number of instances required to establish custom when established by multiple instances?

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There are differing reports regarding the minimum number of instances required to establish custom when repetition is necessary. One narration states that custom is established by two instances because the second instance constitutes a recurrence. Another narration holds that custom is not established unless there are three instances, based on the superficial meaning of the Hadith, the fact that custom is only applied to what is frequent, and the minimum for frequency is three. Additionally, the number three is the maximum considered for repetition in analogous legal matters, such as the days of option in the case of an animal sold while its milk was concealed (al-musarah).