What is the ruling on prostrating on one's hands, that is, placing one's forehead on one's hands during prostration in prayer?

Prostration is not valid if the forehead is placed on the hands instead of directly on the ground, because a condition for the validity of prostration is enabling the forehead to make firm contact with the place of prostration. Placing some prostrating limbs upon others, such as placing the forehead on the hands, is not sufficient, as it leads to an overlapping of the prostrating limbs.