Is it obligatory to repeat the prayer if the Imam says "Allahu Akbar" when rising from bowing, and I was saying "Alhamdulillah" to myself?

The saying "Sami' Allahu liman hamidah" (Allah hears the one who praises Him) is one of the obligatory acts (wajib) of prayer. If someone omits it out of ignorance or forgetfulness, their prayer is valid, but they must perform the prostration of forgetfulness (sujud al-sahw) if they remember or learn its ruling and the interval has not been long. There is no issue for someone who combines both "tasmee'" (saying "Sami' Allahu liman hamidah") and "tahmeed" (saying "Rabbana wa lakal-hamd").