Imam al-Zuhri (may Allah have mercy on him) said: In the time of the Messenger of Allah (peace and blessings be upon him), people used to have debts. We do not know of any free man being sold for a debt. The same as the previous narration is reported from Abu Dawud.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب التفليس / 11276