ولا يجوز بيع السمك في الماء ولا بيع الطير في الهواء
Translation: Selling fish while still in the water and a bird while flying in the air is not permitted in the described form.
The issue is ability to deliver. A seller should not sell an item that cannot realistically be identified, controlled and handed over.
Examples:
1. An uncaught fish swimming in an open river.
2. An uncaptured bird flying free.
A caught fish ready for delivery or an identified bird securely held in a cage is not automatically the same case; its ordinary sale conditions must be assessed separately.