Abu Dawud
The Office Of The Judge
Book Number: 018 / Hadith Number: 3597
Narated By Abdullah ibn Amr ibn al-'As : The Apostle of Allah (pbuh) said: The proviso of a loan combined with a sale is not allowable, nor two conditions relating to one transaction, nor the profit arising from something which is not in one's charge, nor selling what is not in your possession.