Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.066.

Section : Repeating imam’s words for others in the back rows.

If some people cannot hear the imam, it is preferred for one to repeat in a loud voice the imam’s words for the others to hear. There is consensus among scholars that repeating aloud after the imams without there being any real need for it is an abhorrent innovation.