When is the prostration for forgetfulness obligatory?

Question:
The listener, who used the initials (S. Y.) for her name, from the Syrian Arab Republic, Damascus, says: Your Eminence, tell us about the prostration for forgetfulness and the prostration of recitation—may Allah reward you with good—when is it obligatory?

Answer:
The prostration for forgetfulness is obligatory if the imam leaves off an obligatory act of prayer out of forgetfulness, or if he does something forbidden out of forgetfulness; he prostrates for forgetfulness. This is based on the saying of the Prophet (peace and blessings be upon him): So the recommended practice for the imam and the one praying alone, if he does something of what Allah has forbidden in prayer while in a state of forgetfulness, is to prostrate for forgetfulness. For example, if he speaks out of forgetfulness, or adds a unit of prayer out of forgetfulness, or adds a prostration out of forgetfulness, he prostrates for forgetfulness before he says the closing salam. After he finishes the tashahhud and the supplication, he prostrates the two prostrations of forgetfulness. If he prostrates them after the salam, there is nothing wrong with it.
Likewise, if he leaves off an obligatory act out of forgetfulness, such as leaving off saying "Glory be to my Lord, the Most High" in prostration, or "Glory be to my Lord, the Most Great" in bowing, or "My Lord, forgive me" between the two prostrations, he prostrates for forgetfulness before he says the salam. Or if he forgets the first tashahhud and stands up for the third unit, he prostrates for forgetfulness before he says the salam. If he prostrates after the salam, there is no blame on him.

The presenter: May Allah reward you, O Shaykh.