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The Germany international told the club's official website:"I am delighted to be signing for Chelsea, it is a very proud moment for me to be joining this great club.
"I of course want to thank RB Leipzig, the club and the fans, for four fantastic years. You will forever be in my heart. I look forward to next season with my new team-mates, my new manager and of course the Chelsea fans. Together we have a very successful future ahead of us."