When should the prostration of forgetfulness be done, and what should be said in it?

Firstly: 

With regard
to when the prostration of forgetfulness should be done, before or after the
salaam, there is a great difference of opinion among the scholars. The most
correct view is that if a person does something extra in the prayer by
mistake, the prostration must be done after the salaam, and if he omits
something then he should do the prostration before the salaam. If he is not
sure then it is subject to further discussion. If one thing seems more
likely to him than the other then he should do the prostration after the
salaam, and if neither seems more likely then he should do the prostration
before the salaam. This has been discussed in the answer to question no.
12527. 

Secondly: 

It says in
Fataawa al-Lajnah al-Daa’imah (8/7):  

The first
tashahhud in the prayer is one of the obligatory parts of prayer, according
to the more correct of the two scholarly views, because the Prophet
(peace and blessings of Allaah be upon him) used to do it and he said: “Pray
as you have seen me praying.” If he omitted it by mistake, he should
prostrate because of forgetting, and if he omitted it deliberately, then his
prayer is invalid. The one who omitted it by mistake can make up for it by
doing the prostration of forgetfulness before the salaam. End quote. 

Thirdly: 

It is not
prescribed to repeat the tashahhud after the prostration of forgetfulness,
whether it comes before or after the salaam. This has been discussed in the
answer to question no. 7895. 

Fourthly: 

The
prostration of forgetfulness is to be done like the prostration in prayer,
so one should prostrate on seven bones as in the prostration of prayer, and
remember Allaah by reciting the well-known dhikr (Subhaana Rabbiyal-A’la
(Glory be to my Lord most High)) and saying between the two prostrations
Rabb ighfir li, Rabb ighfir li  (Lord forgive me, Lord forgive me).
There is no special dhikr for the prostration of forgetfulness. This is what
has been stated by the scholars. 

Al-Mardaawi
said in al-Insaaf (2/159): 

The
prostration of forgetfulness, what is said during it and after rising from
it are the same as the prostration in prayer. End quote. 

Al-Ramli
said in Nihaayat al-Muhtaaj (2/88): 

The manner
in which it (i.e., the prostration of forgetfulness) is done is the same as
the prostration in prayer, with regard to its obligatory duties and
recommended parts, such as placing the forehead on the ground, being at ease
in the posture, and iftiraash (sitting on the left thigh with the right foot
upwards and its toes pointed towards the qiblah) when sitting in between
them. End quote. 

Some fuqaha’
regard it as mustahabb to say Subhaana man la yas-hu wa laa yanaam
(Glory be to the One Who does not forget or sleep) during the prostration of
forgetfulness but there is not evidence for this. What is prescribed is to
limit oneself to the dhikr that is said when prostrating during prayer, and
not to recite any other dhikr. 

Other
scholarly comments are quoted in the answer to question no.
39399. 

And Allaah
knows best.