what is the episode that Barney recalled that he loved Robin at the first sight?

I remember that there is such a scene, Barney recalled (or told someone) he loved Robin from the beginning.
In that recalled scene, Barney turned around and looked at Robin after he used "Have you met Ted?" to introduce Ted to Robin.
It should be something like Barney recalling how he got attracted to Robin at the first sight and then he had to suffer all the time when he saw Robin sleeping around
But I can't remember what episode it is and I tried to find it but failed. Could someone answer me? THX!!