Abdullah bin Dinar narrates from Ibn Umar (may Allah be pleased with them both) that he said: Pay zakah on the wealth that is in your possession; and a debt that is with a reliable person is like what is in your possession; and wealth that is in the form of a doubtful debt has no zakah on it until it is received.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7624