What is the hadith forbidding bay' al-kali bil-kali?

I was reading a (poorly written) essay recently, wherein a hadith was mentioned but not fully quoted. Allegedly, the classical *fuqaha* used it to forbid *bay' al-kali bil-kali* (Selling of debts for debts). Beyond that, all I know is that it was reported by Ibn 'Umar, as related by al-Tabrani (*sic*). What does this hadith say, and how authentic is it?