Following links the follower’s prayer to the imam’s prayer. Fard behind nafl is not equivalent to nafl behind fard, because the relation of obligation reverses.
In the Hanafi ruling, obligatory prayer cannot be based upon an imam’s voluntary prayer, but the reverse can be valid.
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Whom May One Follow in Prayer? Qualifications and Validity of Imamate
Following links the follower’s prayer to the imam’s prayer. Fard behind nafl is not equivalent to nafl behind fard, because the relation of obligation reverses.
In the Hanafi ruling, obligatory prayer cannot be based upon an imam’s voluntary prayer, but the reverse can be valid.