Here's some stories.[1][2] Of course, these are about future plans, not actually leaving, so take them however you want. AFAIK,the real concern is not that banks will actually "leave" the UK, but that they will downsize as they move operations to other areas of the EU, which makes complete sense. Some of the laws that allowed free access to EU citizens from the UK will end with brexit, and while UK officials will try to get something similar in place, there are EU officials that feel that a deal that's essentially the same is not in their favor, and that the UK should pay a penalty for leaving. This is the uncertain climate that is causing some banks to hedge their bets and move some operations to the EU.
1: http://uk.reuters.com/article/uk-britain-eu-banks-idUKKCN12M...
2: https://www.theguardian.com/politics/2016/oct/22/leading-ban...