It is sinful for someone to walk in the place of prostration of the worshiper on the ground without a barrier, and if it is outside of the place of prostration, it is not sinful even without a barrier. This applies if the prayer is in a large mosque or in the desert. However, in a small mosque, it incurs sin; because the small mosque is considered one place, so wherever the worshiper is, it is regarded as the place of prostration. The small mosque is defined as being less than sixty cubits, and it is said to be forty. Refer to: Majma' al-Anhar 1: 121.