Source: https://bitcoin.tax/faq

Do I need to file or pay taxes if I own Bitcoins?

Not if you just bought Bitcoins (or any crypto-currencies) with your own money. However, if you traded, sold, or used any to purchase something, then you might. If you were given Bitcoins as payment, as a salary, or as a gift/donation, this is income and should be reported as any other income you earn.

I didn't move any money out of the exchange, I just bought back in. What about then?

Yes. A tax event occurred and you gained money, even though it isn't in your bank account. For tax purposes it is treated no differently.