Praise be to Allah.
Passing a verdict on a accused person whose case has been brought to court and stating whether he is guilty or not guilty is regarded as ruling and passing judgement. One of the conditions of ruling and passing judgement is that it should be based on the Book of Allaah and the Sunnah of His Prophet (blessings and peace of Allaah be upon him) even if it has to do with passing judgement on kuffaar or judging between them.
Allaah says (interpretation of the meaning):
“So if they come to you (O Muhammad صلى الله عليه وسلم), either judge between them, or turn away from them. If you turn away from them, they cannot hurt you in the least. And if you judge, judge with justice between them. Verily, Allaah loves those who act justly”
“And so judge (you O Muhammad صلى الله عليه وسلم) among them by what Allaah has revealed and follow not their vain desires, but beware of them lest they turn you (O Muhammad صلى الله عليه وسلم) far away from some of that which Allaah has sent down to you. And if they turn away, then know that Allaah’s Will is to punish them for some sins of theirs. And truly, most of men are Faasiqoon (rebellious and disobedient to Allaah)”
Shaykh ‘Abd al-Rahmaan al-Sa’di (may Allaah have mercy on him) said:
“And so judge (you O Muhammad صلى الله عليه وسلم) among them by what Allaah has revealed” -- this verse is the one which is said to abrogate the verse “either judge between them, or turn away from them”. But the correct view is that it does not abrogate it, and that this indicates that the Prophet (blessings and peace of Allaah be upon him) was given the choice between passing judgement between them or not doing so, and that was because their aim was not to refer to the truth for judgement. This verse indicates that if he did pass judgement, then he was to pass judgement between them in accordance with that which Allaah had revealed of the Qur'aan and Sunnah, and this is the justice which is referred to above, in the verse in which Allaah says: “And if you judge, judge with justice between them”. This points to what is meant by justice, and that it should be based on the rulings that Allaah has prescribed, which include the ultimate justice and fairness. Anything contrary to that is injustice and wrongdoing. End quote.
Tafseer al-Sa’di, p. 234
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said:
No one has the right to judge between any of Allaah's creation, whether between Muslims or kuffaar or young men or armies or the poor or anyone else, except in accordance with the rulings of Allaah and His Messenger. Whoever seeks anything other than that is included in the words of Allaah (interpretation of the meaning): “Do they then seek the judgement of (the days of) Ignorance? And who is better in judgement than Allaah for a people who have firm Faith” [al-Maa’idah 5:50] and “But no, by your Lord, they can have no Faith, until they make you (O Muhammad صلى الله عليه وسلم) judge in all disputes between them, and find in themselves no resistance against your decisions, and accept (them) with full submission” [al-Nisa’ 4:65]. End quote.
Majmoo’ Fataawa Ibn Taymiyah, 35/407, 408.
The scholars of the Standing Committee for Issuing Fatwas said:
It is not permissible for a Muslim to work as a judge except in sharee’ah courts which apply the laws of Allaah in all their rulings. End quote.
Shaykh ‘Abd al-‘Azeez ibn Baaz, Shaykh ‘Abd al-‘Azeez Aal al-Shaykh, Shaykh Saalih al-Fawzaan, Shaykh Bakr Abu Zayd.
Fataawa al-Lajnah al-Daa’imah, 12/136, 127
It is no secret to anyone that the reference point for passing judgement on people in the country asked about and -- unfortunately -- in many Muslim countries is not the Qur'aan and Sunnah. Based on that, the starting point for passing judgement on people -- both Muslims and kaafirs -- is man-made laws, which are opposed to the laws of the Lord of the Worlds. Based on that, it is not permissible to serve on the jury or work in any other part of the courts which judge between people on the basis of something other than that which Allaah has prescribed.
And Allaah knows best.