Board Thread:General Discussion/@comment-26901329-20150805102157/@comment-25926288-20150805145004

Well, they said it in season 2.

http://www.wetpaint.com/once-upon-a-time-season-2-spoiler-can-rumple-still-see-the-future-adam-horowitz-says-767275/

The tweet was March 2013 (Midseason 2). Season 3 started in September of that year.