Is it obligatory to fast six days – three for Umrah and three for Hajj – or only three days, due to wearing underwear during the Umrah ihram and also during the Hajj ihram for a Mutamatti who has exited ihram in between?

The expiation (fidyah) is obligatory for whoever commits one of the prohibitions of ihram while knowing its prohibition. However, for one who does not know, there is nothing upon him. Whoever commits a prohibition out of necessity is not sinful, but the fidyah is upon him. As for one who does it forgetfully, ignorantly, or under duress, there is nothing upon him, nor is there a fidyah. But whoever commits it without necessity or excuse is sinful and the fidyah is upon him.

The fidyah for harm—excluding sexual intercourse and hunting—is an option between: sacrificing a sheep, or feeding six poor persons (half a sa‘ for each poor person), or fasting for three days. Whoever chooses to fast may fast anywhere. As for one who chooses to feed or sacrifice a sheep, he must do so in the Haram or in the place where the prohibition was committed.

If the prohibition is committed in both 'Umrah and Hajj, then two expiations are obligatory: one fidyah for 'Umrah and one fidyah for Hajj. If he chooses fasting as expiation, he must fast for six days (three days for 'Umrah and three for Hajj).