Is it true that some people say "صدق الله العظيم" (Allah the Almighty has spoken the truth) upon hearing the Adhan?

It is recommended for the one who hears the Adhan to repeat what the Muezzin says, then to send prayers upon the Prophet (peace be upon him). It is not from the Sunnah to say, "Allah, the Most High, has spoken the truth," in response to the Muezzin. Rather, it is an added innovation, because dhikr (remembrance of Allah) is not prescribed to be restricted by a specific time, place, or number, in a way that makes it Sunnah, except with evidence.