Assalaamu alaykum. Please, I need a clarification on the ruling of saying ‘Allaahu Akbaar’ instead of ‘Allaah Akbar’ during prayer because I sometimes hear some people, while praying, occasionally say ‘Allaahu Akbaar’ instead of ‘Allaahu Akbar’. May Allah grant us Paradise.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

The scholars advised that expanding the letter 'ba' in the word Akbar in Takbeer (to be Allaahu Akbaar instead of Allaahu Akbar) is a major error and that if the praying person makes this mistake in the opening Takbeer of the prayer, the prayer is invalid because this error alters the meaning. The Arabic word “Akbaar” is the plural form of "kabar" which means  a drum.

Whenever you hear someone make such a mistake in the prayer, you should advise him and clarify his mistake to him if possible.

Allaah knows best.