Under which condition is a worshipper permitted to level the pebbles at their place of prostration?

Disliked Acts in Prayer (مكروهات الصلاة) Part 1: Physical Actions

• There is no permission under any circumstances.
• If prostration is physically impossible, they may level it only once.
• They can level it as many times as they want in every unit.
• Only after they have finished the entire prayer.