If a person does not do the obligatory prostration of forgetfulness
Question

                If a person did not do the prostration of forgetfulness for omitting an obligatory part of the prayer, what should he do, if he is praying alone or with an imam?.
Praise be to Allah.
    If a worshipper deliberately omitted the prostration of 
    forgetfulness for omitting an obligatory part of the prayer, then his prayer 
    is invalid, just as if he deliberately omitted any obligatory part of the 
    prayer. 

    But if he forgot to do the prostration of forgetfulness, if 
    he remembers it after saying the salaam he should do it, unless a long time 
    has passed or he has left the mosque, in which case he does not have to do 
    anything, and his prayer is valid. 

    Shaykh Ibn ‘Uthaymeen was asked about a man who forgot the 
    first tashahhud, and he knew that he had to do the prostration of 
    forgetfulness before the salaam, but he forgot and said the salaam. What is 
    the ruling? 

    He replied: If he remembers within a short time, he should do 
    the prostration, but if a long time has passed, then it is waived, such as 
    if he did not remember until after a long time, if he had left the mosque 
    then he does not have to go back to the mosque, and it is waived in his 
    case. End quote. 

    Fataawa Ibn ‘Uthaymeen (14/50). 

    If the imam forgets to do the prostration of forgetfulness, 
    the person praying behind him should not do it on his own. Rather he should 
    say the salaam with the imam, then remind him after the salaam to do the 
    prostration of forgetfulness, then the imam should prostrate and the person 
    praying behind him should prostrate too. 
And Allaah knows best.