What is the ruling for someone who forgets the prostrations of forgetfulness (sujood as-sahw) in prayer? Does their prayer become invalid as a result? How can this be rectified? And is it obligatory to interrupt a supererogatory prayer (sunnah prayer) to make it up?

If one forgets the prostration of forgetfulness (sujud al-sahw) and says the salam (finishing prayer), he must perform it if the time is short. This is the well-known opinion. However, if a long time has passed or he has left the mosque, it is waived, and his prayer is valid. Some scholars are of the opinion that he should prostrate even if a long time has passed or he has left the mosque, because it rectifies a deficiency. Others hold that he should prostrate even if he has left the mosque, provided the time elapsed is short.