Musannaf Ibn Abi Shayba 13:1207

Abu Bakr narrated to us, he said: Ibn Mahdi narrated to us, from Abu Shihab, from al-Ala’ ibn al-Musayyab, from al-Hakam, who said: "The *mudarabah* (capital) may be exchanged for a debt, but a debt may not be exchanged for *mudarabah*."