Mail from Amazon that my password has been changed

I got a mail today from Amazon that my password has been changed due to security reasons.

".... As part of our routine monitoring, we discovered a list of email address and password sets posted online. While the list was not Amazon-related, we know that many customers reuse their passwords on several websites. We believe your email address and password set were on that list. So we have taken the precaution of resetting your Amazon.com password..."

Any idea, which list are they talking about?