From Sahih Bukhari (Hadith 2256):
Narrated `Abdullah:The people used to sell camels on the basis of Habal-al-Habala. The Prophet (ﷺ) forbade such sale. Nafi` explained Habal-al-Habala by saying. "The camel is to be delivered to the buyer after the she-camel gives birth
Reference: Bukhari 2256