You don't have to report that you reinvested dividends in the year that they are paid. You would see what dividends were reinvested on your brokerage statement, or your statements from the individual companies in which you own stock.

If you own stock and reinvest the dividends, you report the dividends as income in the year they are paid. They are used to purchase shares or fractional shares of the underlying stock. When you sell shares, these dividends on which you paid tax, become part of the cost basis of the shares, and are used to reduce your gain/increase your loss on the sale.

See reinvested dividends in this link about Cost Basis.