In sajdah, one must place his two big toes on the ground. However, it is not necessary to place the tips of the toes on the ground; rather, placing the back or front of them also suffices. If a person does not place his big toe on the ground but instead places his other toes or the top of his foot on the ground, or if on account of having long nails his big toe does not make contact with the ground, his prayer is invalid.
If one has performed prayers in this manner while not knowing the ruling due to his own fault, he must perform them again.