Sahih Bukhari
Transferance of a Debt from One Person to Another (Al-Hawaala)
Volume: 003 / Book Number: 037 / Hadith Number: 486
Narrated Abu Huraira:
The Prophet said, "Procrastination (delay) in paying debts by a wealthy man is injustice. So, if your debt is transferred from your debtor to a rich debtor, you should agree."