Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.016.
Section : Zakah on Banknotes and bonds.
Since they are documents with guaranteed credits, banknotes and bonds are subject to zakah once they attain the minimum of nisab – that is, a person may change them into currency immediately. The minimum of nisab is twenty-seven Egyptian riyals.