I had a quick look though the code, I couldn’t find an exact match that is output by Discourse. I might be missing something though.
However there are the following messages which seem pretty close:
not_activated: "You can't log in yet. We previously sent an activation email to you at <b>{{sentTo}}</b>. Please follow the instructions in that email to activate your account."
not_activated: "You can't log in yet. We sent an activation email to you. Please follow the instructions in the email to activate your account."
activate_email: "<p>You're almost done! We sent an activation mail to <b>%{email}</b>. Please follow the instructions in the email to activate your account.</p><p>If it doesn't arrive, check your spam folder, or try to log in again to send another activation mail.</p>"