The source is supposedly a "respected virologist" from the Charite hospital in Berlin. He started a daily podcast related to COVID-19 [1], but I did not have a chance to find this quote yet.

EDIT: That same virologist also warned about a "second wave" of cases starting this fall, after a reduction during summer, due to seasonal strength of the immune system.

[1] https://www.youtube.com/playlist?list=PLkKON9te6p3OpxqDskVsx...