Wages (Kitab Al-Ijarah)
Dawud :: Book 23 : Hadith 3495
Narrated Abdullah ibn Amr ibn al-'As:
The Apostle of Allah (peace_be_upon_him) 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.