IRS Publication 525 Taxable and Nontaxable Income, page 33:

Gifts and inheritances. In most cases, property you receive as a gift, bequest, or inheritance isn't included in your income. However, if property you receive this way later produces income such as interest, dividends, or rents, that income is taxable to you. If property is given to a trust and the income from it is paid, credited, or distributed to you, that income is also taxable to you. If the gift, bequest, or inheritance is the income from the property, that income is taxable to you.

Inherited pension or IRA. If you inherited a pension or an IRA, you may have to include part of the inherited amount in your income. See Survivors and Beneficiaries in Pub. 575 if you inherited a pension. See What if You Inherit an IRA? in Pubs. 590-A and Pub 590-B.