Sahih Muslim
The Book of Transactions (Kitab Al-Buyu)
Book Number: 010 / Hadith Number: 3796
Abu Huraira (Allah be pleased with him) reported Allah's Messenger (may peace be upon him) as saying: Delay (in the payment of debt) on the part of a rich man is injustice, and when one of you is retired to a rich man, he should follow him.