Is there an authentic hadees that proves the existence of this method from the times of the beloved prophet p.b.u.h?
Re: Congregational Dua
Peace Muzna,
As far as I'm aware, there is no hadith pertaining to 'Congregational dua' made right after end of any Salah. However, it is common among those who follow teaching of 'Hanafi School of Thoughts'.
Almighty Allah knows best.