What is the required expiation (Kaffarah) for a husband who has performed Zihar against his wife?

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If the husband has performed Zihar against his wife, he is prohibited from having marital relations with her until he performs the expiation. The expiation is restricted solely to feeding the needy, because he does not possess a Muslim slave whose freedom could be the means of expiation, nor can he purchase one, and performing the fast (Siyam) is not valid for him.