Abu Dawud
Wages
Book Number: 017 / Hadith Number: 3495
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.