Placing the Forehead at a level higher than the Feet in Prostration

What is the permissible height above the feet at which the forehead could be placed during prostration. Answer: Half arm’s length. Approx 9 inches. When the rows are too close to each other making sajdah on the floor impossible, one can place their forehead on the back of the person in front of you who is praying the same salaah.