Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.050A.

Most scholars are of the opinion that khums is due on anyone who finds a treasure, whether he happens to be a Muslim, a free non-Muslim subject (zhimmi), old, young, sane, or insane. However, the guardians of the young and insane must pay it on their behalf. Ibn al-Munzhir comments that all learned persons agree that a zhimmi who finds rikaz has to pay its khums. This is also the opinion of Malik, the scholars of Madinah, ath-Thauri, al-Auza’i, the scholars of Iraq, those who use analogy (ashab ar-ra’y), and others. Ash-Shaf’i stated that khums is only due upon those who must pay zakah.