سنن أبي داود

Narrated 'Amr b. Suh'aib:

On his father's authority, said that his grandfather told that the Messenger of Allah (ﷺ) forbade the type of transactions in which earnest money was paid.

Malik said: This means, as we think--Allah better knows-that a man buys a slave or hires an animal, and he says: I give you a dinar on condition that if I give up the transaction or hire, what I gave you is yours.