Maliks Muwatta Book 31, Hadith Number 85.
Section : Debts and Transfer Debts in General.
Yahya related to me from Malik from Abu’z-Zinad from al Araj from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, “Delay in payment by a rich man is injustice, but when one of you is referred for payment to a wealthy man, let him be referred.”