Sahih Bukhari
Merits of the Helpers in Madinah (Ansaar)
Volume: 005 / Book Number: 058 / Hadith Number: 183
Narrated Ibn 'Umar:
In the pre-lslamic period of ignorance the people used to bargain with the meat of camels on the principle of Habal-al-Habala which meant the sale of a she-camel that would be born by a she-camel that had not yet been born. The Prophet forbade them such a transaction.