Abu Dawud Book 17, Hadith Number 3495.

Chapter : Not known.

Narated By Abdullah ibn Amr ibn al-‘As : The Apostle of Allah (PBUH) 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.