The amount you pay to buy a CD generally is not taxable, even when you cash it in. However, you will be responsible for paying taxes on any interest you earned from the CD before it matured. This interest must be reported to the IRS as taxable income.
According to the IRS, interest is taxable in the year it is paid.
If your CD earns more than $10 of interest in a year, the bank or credit union should send you a 1099-INT statement. You can see how much interest you earned that year in box 1.