When did Wally find out Robin's secret identity?

i have watched every episode and read every comic and i can't find anything. It seems that oficially us, the audience confirmed he knew his secrect identity when the episode "Image" came out. I think that in a comic that was given on free comic book day mentioned that Wally knew Robin identity, but i don't have that comic. When did Wally find out? is what i'm asking, and where can i read or watch about it.