Uthman bin 'Umar bin Fars عثمان بن عمر بن فارس · The Chapters on Charity

“I asked Anas bin Malik: 'What if a man gives his brother a loan, then (the borrower) give him a gift?’ The Messenger of Allah (ﷺ) said: ‘If anyone of you borrow something then he gives (the lender) a gift or gives him a ride on his riding-beast, he should not accept the gift or the ride, unless they used to treat each other in that manner beforehand.'”