Reciting du’aa’ for forgiveness in unison following prayer

Praise be to Allah.

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.