Where? Here's the transcript of the initial announcement [1]. No mention of open source that I see.
Dr. Dobb's had an article in 2014 [2] which said "Swift is proprietary and closed: It is entirely controlled by Apple and there is no open source implementation."
EDIT: I found a HN post from 2015, just over a year after Swift was released, titled "Swift will be open source later this year" [3]. Not immediate at all.