When the imam finishes praying he recites the du’aa’ for forgiveness (istighfaar) in communal fashion, out loud. Is this permissible?.
It says in Fataawa al-Lajnah al-Daa’imah:
Saying du’aa’ out loud following the five daily prayers and the regular Sunnah prayers, or reciting du’aa’ after the prayers in unison as a regular practise is a reprehensible innovation (bid’ah), because it is not reported that the Prophet (peace and blessings of Allaah be upon him) or his companions (may Allaah be pleased with them) did anything like that.
Fataawa Islamiyyah, 1/319
See also the answer to Question no. 10491 for more information.