How is acknowledgment of adultery (*Iqrar bi al-Zina*) established?

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There are two narrations concerning the establishment of adultery solely through acknowledgment (*Iqrar*), mentioned by Abu Bakr. Imam Al-Shafi'i holds two opinions: one, that it is established by two witnesses, analogous to all other acknowledgments; the second, that it requires four witnesses because it results in a *Hadd* (prescribed punishment), resembling the act itself.