When is accepting a banquet invitation obligatory?

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The acceptance of an invitation is only obligatory if the source of the food is lawful (tayyib) and no forbidden act is witnessed at the gathering. Ahmad explicitly stated this condition. If the food is derived from illicit earnings, acceptance is not obligatory because its preparation and consumption are themselves reprehensible acts, making abstention preferable. If attendance occurs nonetheless, consumption of that food is not permissible.