Maliks Muwatta Book 17, Hadith Number 18.

Section : The Zakat on Debts.

Yahya related to me from Malik from Ayyub ibn Abi Tamima as-Sakhtayani that Umar ibn Abd al-Aziz, when writing about wealth that one of his governors had collected unjustly, ordered it to be returned to its owner and zakat to be taken from it for the years that had passed. Then shortly afterwards he revised his order with a message that zakat should only be taken from it once, since it was not wealth in hand.