Routh eventually played recurring character Ray Palmer (The Atom) on other DC Comics-related projects, Arrow and The Flash. (In The Flash, characters even make veiled references to his time as Superman.)
When did that reference to Superman Returns occur in The Flash and what was that reference?
Upon Ray Palmer's arrival Caitlin and Cisco have an exchange when they see something flying in the air. First asking if it's a bird, and then a plane, and Felicity replies that it's her boyfriend. This is a reference to the iconic Superman phrase, referencing Brandon Routh's former title role as Superman in the 2006 film adaptation Superman Returns.
and this happened in the only flash episode Ray Palmer (Atom) appeared in, s01e18, All Star Team Up