Musnad Abu Ya'la al-Mawsili 117:203

Ammar narrated to us, Yusuf narrated to us, Thabit narrated from Anas that a funeral was brought to the Prophet (peace and blessings of Allah be upon him) for him to pray over it. He said, "Does he have any debt?" They said, "Yes." The Prophet (peace and blessings of Allah be upon him) said, "Indeed, Gabriel has forbidden me from praying over a man who has a debt upon him, and he said: 'The owner of the debt is held hostage in his grave until his debt is paid off for him.'" So the Prophet (peace and blessings of Allah be upon him) refused to pray over him.