The 22-year-old has been a key player for Borussia Dortmund this season.
“I am very proud to take this step,” he told reporters at a press conference. “It is very difficult. I am a Borussian and will always remain a Borussian.
“I will miss Dortmund, I will miss the Bundesliga and I will miss my home country. I want to thank all who supported me on my way.”
He continued: “Saying goodbye to my team was very hard for me. They’re in my heart and the farewell was very emotional. Not so many clubs were an option for me, because Dortmund is so precious.
“Everybody in this team has given everything for the other. This is an outstanding team that won a great title.”
“Money hasn’t played any role in this move,” he added. “I’ve taken this decision only because of sporting reasons. I’m not one to lie. The fans knew how I would decide.”
