The people used to sell camels on the basis of Habal-al-Habala. The Prophet ﷺ forbade such sale. Nafiʿ explained Habal-al-Habala by saying. "The camel is to be delivered to the buyer after the she-camel gives birth."  

كَانُوا يَتَبَايَعُونَ الْجَزُورَ إِلَى حَبَلِ الْحَبَلَةِ فَنَهَى النَّبِيُّ ﷺ عَنْهُ فَسَّرَهُ نَافِعٌ أَنْ تُنْتَجَ النَّاقَةُ مَا فِي بَطْنِهَا