My source is an interview in the first German state TV station (ARD). I am sorry that I can't provide you with an English source. If you understand German, please watch https://youtu.be/IHF0IDtW6LA?t=882 from minute 14:42.
As a second source, I found a written report on the TV talk here, for which you may use, e.g., deepl.com to translate it: https://www.express.de/news/promi-und-show/corona-talk-spd-p...