Pretty sure it's written on peyton's closet door. Not sure if they ever actually say it, but they refer to it being written on her door in episode 5.06
posted over a year ago
Yeah. It's written on Peyton's old bedroom door (shown in season 5)
Yeah, they said it. She was telling Lucas that she went by her old house and she said "you know what it said? Lucas and Peyton, true love always, ALWAYS Lucas. That's what we were supposed to have."
It didn't exist for the first 4 seasons. In the fifth season, the writers kind of went back and re-wrote history and said it was written on the door in Peyton's room. And then since then they mentioned it at least one other time (it probably was more than once but right now I can only remember when Peyton referenced it in a fight with Lucas in season 5). It wasn't brought up because it was something they said... the way "always and forever" was part of Naley's wedding vows.