Is saying "May Allah accept [it]" to someone who finishes their prayer unlawful and an innovation?

Committing to a Muslim saying to his brother after prayer, "May Allah accept [it from you]," with the intention of drawing near to Allah and as an act of worship, is an innovation (bid'ah). This is because it was not reported from the Prophet (peace be upon him) nor from his Companions. The fundamental principle in acts of worship is restriction to what has been legislated (al-tawqif), so whoever introduces something into the religion that is not part of it, it is rejected. However, if it is said occasionally without the belief of drawing near to Allah (qurbah), then there is no harm in it. Similarly, there is no harm in saying it on Eid day, as the Companions used to say it to one another on Eid day.