I tried to find it and failed. I'm sure a Sera fan here can hook you up though!
It is an ambient dialogue after you have started a romance with Sera. I am paraphrasing, but it goes something like Solas commenting [favorably] on the relationship, in which Sera replies something like: 'Fancy her, do you?' He doesn't respond to her, and just continues to say that she [Sera] made the natural choice.
I went searching and I think this is the one (for those who don't want to sift through the 6 hours of banter
):
Solas: I am not surprised you were drawn to the inquisitor, Sera.
Sera: Why? You fancy her too?
Solas: I meant that it seems natural that you would desire another elf
Sera: Hey Inky. Was I jumping you full on, or did I say something about you being elfy?
Inq: You were, hesitant, about me.
Sera: I know right! And you're so good. She really is.
Solas: You were that concerned she was your own kind?
/
Inq: That's personal, I'm not getting into it
Sera: He made it a thing
/
Inq: Quiet, both of you
Solas: As you would have it.
/
Sera: Nothing? She's shy like that. Point is I'm with her because she's her, not because she's elfy.
Solas: You dislike your own kind so much?
Sera: My own kind can be a bit of an arsehole.