The prisoner must pay zakaah on his wealth
Question

                If a person is imprisoned and he has wealth, does he have to pay zakaah?.
Praise be to Allah.
    If a person is kept away from his wealth, such as if he is a 
    captive or imprisoned, then he must pay zakaah because he is the owner of 
    the wealth. 

    Ibn Qudaamah (may Allaah have mercy on him) said in 
    al-Mughni (4/275): If the owner is taken captive, zakaah is not waived 
    in his case, whether he is kept away from his wealth or not, because his 
    disposal of his wealth is still valid and he may sell it, give it away or 
    appoint someone to handle it on his behalf. 

    Al-Nawawi (may Allaah have mercy on him) said in 
    al-Majmoo’ (5/316): If the owner of the wealth is taken captive, and is 
    kept away from his livestock, there is a difference of scholarly opinion 
    concerning the matter. The more correct view is that zakaah must be paid, 
    because his disposal of his wealth is still valid. Our companions said: 
    Whether he is being held captive by kuffaar or Muslims. End quote. 

    Al-Bahooti (may Allaah have mercy on him) said in 
    Kashshaaf al-Qinaa’ (2/176): If the owner of the wealth is taken captive 
    or imprisoned, and is prevented from disposing of his wealth, zakaah is not 
    waived because he is still the owner. 

    And Allaah knows best.