Abdullah bin Dinar narrates from Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) forbade bay' al-kali' bi-l-kali' (that is, selling a debt for a debt).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10539