He forgot and said Allaahu akbar instead of Sami’aAllaahu liman hamidah

Saying Sami’a Allaahu liman
hamidah, for both the imam and the one who is praying alone, is one of the
obligatory parts of prayer, according to the correct opinion, as was stated
in the answer to question no. 43574. 

The one who forgets to say
that and does not say anything, or says Allaahu akbar instead of that, and
remembers whilst he is still praying, should do the two prostrations of
forgetfulness before saying the salaam. 

The basic principle is that
the one who forgets an obligatory part of prayer or says a prescribed phrase
– such as Allaahu akbar – in the wrong place, should do the prostration of
forgetfulness. 

It says in Daleel
al-Taalib: The prostration of forgetfulness is required if one adds a
bow or prostration or standing or sitting, even if it is as long as the
sitting between the two prostrations, or he says the salaam before
completing the prayer, or he makes a mistake in recitation that alters the
meaning, or he omits an obligatory part. End quote. 

And Allaah knows best.