Is it valid to raise one foot or both feet during prostration?

The questioner:
He says: Honorable Shaykh, may Allah grant you success. It is observed in many mosques that some of the ma'mum (those praying behind the imam) raise one foot or both during prostration. Should one tell him that his prayer is not valid and order him to repeat it, even though he is ignorant?

The Shaykh:
If he raises his foot during all prostrations and never places it down, and he is not excused, he is ordered to repeat the prayer because he left a pillar of the prayer. As for if he raises it and then returns it and places it on the ground before rising from the prostration, this is considered a deficiency in a valid prayer, because he placed his feet and returned the foot to the ground, so the placement of the feet on the ground was achieved. However, his raising it during part of the prostration is a flaw and a deficiency.

The questioner:
Honorable Shaykh, and what about one who was forgetful, not ignorant, and did not place either of his feet on the ground during prostration? Does he have to repeat the prayer?

The Shaykh:
Forgetfulness and ignorance do not drop the obligation; they only drop the sin. If one forgets to pray, is the prayer dropped from him? No. If one forgets to perform wudu' (ablution), is the wudu' dropped from him? No. But the sin is dropped from him. As for the obligation, he must fulfill it, and forgetfulness and ignorance do not drop it.