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

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 Allaah.
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 inal-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.