Is the prayer of one who forgot to say "Rabbana wa lakal-hamd" (Our Lord, and to You be all praise) in one of the rak'ahs, then forgot to perform the two prostrations of forgetfulness (sujud al-sahw) after the tashahhud and salam, and remembered immediately after the salam, then said "Allahu Akbar" and performed the two prostrations of forgetfulness and said the salam without a tashahhud, valid or not?

Scholars have differed regarding the prostration of forgetfulness (sujud al-sahw) when omitting the takbirat al-intiqal (transition takbirs) or the saying: "Rabbana wa lakal-hamd" (Our Lord, and to You be all praise). Some, like the Hanbalis, consider it obligatory, while others, like the Shafi'is, do not see the prostration as necessary for it. The more preponderant opinion is that sujud al-sahw is prescribed for omitting this dhikr (remembrance). Your prostration after the taslim (salutation of peace) is valid and does not invalidate the prayer. The tashahhud (testimony of faith) is not prescribed after sujud al-sahw.