The Institutions of State in the Khilafah by Hizb ut-Tahrir

Anyone taking up the post of judge must be a Muslim, free, mature, sane, just, a Faqih (learned scholar), and aware of how to apply the Islamic rulings to the reality (li-tanzil al-ahkam ‘ala ’l-waqi’). As for the person who takes up the judiciary of Mazalim, in addition to the conditions mentioned, he must also be male and a Mujtahid (legist), just like the supreme judge (Qadi al-Qudah), because this post is in fact judiciary and ruling, for he judges over the ruler and executes the Shariah upon him. Therefore he must be male in addition to the other conditions of the post of judge; one of which is to be a Faqih. Furthermore, he has to be a Mujtahid, because part of the Mazalim which he may be required to look into is whether the ruler has ruled by other than that which Allah has revealed, i.e. ruled by a law that has no Shariah evidence to back it, or in case the evidence he used does not relate to the event, and only the Mujtahid can deal with such Madhlima. Therefore if he were not a Mujtahid, he would be judging without having knowledge; and that is forbidden. Therefore, in addition to the conditions of the ruler and those of the judge, he should also be a Mujtahid.