*Question #6: (a) “What is the general rule of the sujūd us-sahū (prostrations of forgetfulness)?”
Shaykh al-Albānī answers: “(It is due upon a person) under any circumstances if (something was done or not done in prayer) out of forgetfulness, even if it is a sunnah.”
*Question #12: (b) “Is the sujūd us-sahū before the salām or after the salām?”
Shaykh al-Albānī answers: “Regarding every sujūd us-sahū, its doer has the choice between prostrating before the salām or after the salām. And the detailed explanation that is mentioned in some books, especially the Hanbali ones, that (the sujūd us-sahū) is before the salām for making additions (in the prayer), and after the salām for omissions, this is baseless. Those who say it, they saw some occurrences to which the likes of this detail applied, but there are (also) occurrences that nullify this detail. Therefore, a person has the choice between 1-making the taslīm then prostrating, and 2-not making the taslīm and prostrating then making the taslīm of exiting the prayer.”
*Question #7: (c) “When is the sujūd us-sahū, before the salām or after it?”
Shaykh al-Albānī answers: “The one who forgets in prayer has a choice: if he wants, he makes the taslīm (first) and that is absolutely better; and if he wants, he does not make the taslīm except after completing the two prostrations of forgetfulness.”
*Question #10: (d) “What does the one who forgets (in prayer) say during the sujūd us-sahū?”
Shaykh al-Albānī answers: “The same as the usual (dhikr[1] said during prostrating and sitting in-between the prostrations); i.e., there isn’t anything special.”
*Question #3: (e) “Does the prayer become invalid if the sujūd us-sahū is left out on purpose?”
Shaykh al-Albānī answers: “If he intentionally leaves out the sujūd us-sahū, the prayer does not become invalid. But, he will be a sinner because of his leaving out the two prostrations of forgetfulness that the Messenger (of Allaah) (صلى الله عليه وسلم) has ordered.”
~ asaheeha translations ~
[1] words of remembrance of Allāh
Source: https://www.islamland.com/eng/articles/the-prostrations-of-forgetfulness