Maliks Muwatta Book 17, Hadith Number 17.
Section : The Zakat on Debts.
Yahya related to me from Malik from Ibn Shihab from as-Sa’ib ibn Yazid that Uthman ibn Affan used to say, “This is the month for you to pay your zakat. If you have any debts then pay them off so that you can sort out your wealth and take the zakat from it.”