He is not sure whether he said Rabbana wa laka’l-hamd after the imam said the takbeer for prostration
Question

                I was praying with the imam, and whilst I was praying the imam said “Sami’a Allaahu liman hamidah” and after he said the takbeer for prostration, I was not sure whether I said Rabbana wa laka’l-hamd or not. Is my prayer valid, or do I have to repeat it?.
Praise be to Allah.Firstly: 
Tasmee’ (which means saying 
    Sami’a Allaahu liman hamidah) when rising from bowing, and tahmeed (which 
    means saying Rabbana laka’l-hamd) when standing up straight are Sunnah 
    mustahabbah according to the majority of scholars, but the Hanbalis are of 
    the view that they are obligatory, which is the correct view. 
Shaykh Ibn ‘Uthaymeen (may 
    Allaah have mercy on him) said in al-Sharh al-Mumti’ (3/433): 
The evidence for that 
    (i.e., that it is obligatory) is as follows: 
(i)
The Messenger  (peace 
    and blessings of Allaah be upon him) always did that, and he did not omit to 
    say Sami’a Allaahu liman hamidah in any circumstances. 
(ii)
It is the sign for moving 
    from bowing to standing. 
(iii)
The Prophet  (peace and 
    blessings of Allaah be upon him) said: “When he says Sami’a Allaahu liman 
    hamidah, then say Rabbana wa laka’l-hamd.” End quote. 
See the answer to question 
    no. 102700. 
Secondly: 
If a person is not sure 
    whether he omitted one of the obligatory duties of prayer during the prayer, 
    then he is like one who omitted it. If he is still at that point in the 
    prayer, then he should do it, and he does not have to do anything else. If 
    he has passed that point in the prayer then it is not possible for him to do 
    it, but he has to do sujood al-sahw (the prostration of forgetfulness). 
Based on this, if your 
    doubt and uncertainty occurred whilst you were standing up straight after 
    bowing, then you should have said these words then, because you were still 
    at that point in the prayer. 
If the uncertainty came to 
    you after you prostrated, then the time for saying tahmeed had passed, and 
    in that case you were like one who omitted it out of forgetfulness. If you 
    had caught up with the first rak’ah with the imam, then you should have said 
    the tasleem with the imam and the prostration of forgetfulness would have 
    been waived for you in that case. 
If you had come late and 
    missed one or more rak’ahs with the imam, then you should do the prostration 
    of forgetfulness before the tasleem, after completing your prayer. If you 
    did not do the prostration for forgetfulness in this case because you did 
    not know that it is obligatory, then your prayer is still valid and you do 
    not have to do anything. 
To sum up: your prayer is 
    valid and you do not have to repeat it. As for the prostration of 
    forgetfulness, it is according to the details mentioned above. 
And Allaah knows best.