If an estate return form 1041 must be filed then it would be reported there as :
Income in respect of a decedent.
When completing Form 1041, you must take into account any items that are income in respect of a decedent (IRD).
In general, IRD is income that a decedent was entitled to receive but that was not properly includible in the decedent's final income tax return under the decedent's method of accounting.
IRD includes:
All accrued income of a decedent who reported his or her income on the cash method of accounting,
Income accrued solely because of the decedent's death in the case of a decedent who reported his or her income on the accrual method of accounting, and
Income to which the decedent had a contingent claim at the time of his or her death.
Some examples of IRD for a decedent who kept his or her books on the cash method are:
Deferred salary payments that are payable to the decedent's estate,
Uncollected interest on U.S. savings bonds,
Proceeds from the completed sale of farm produce, and
The portion of a lump-sum distribution to the beneficiary of a decedent's IRA that equals the balance in the IRA at the time of the owner's death. This includes unrealized appreciation and income accrued to that date, less the aggregate amount of the owner's nondeductible contributions to the IRA. Such amounts are included in the beneficiary's gross income in the tax year that the distribution is received.
The IRD has the same character it would have had if the decedent had lived and received such amount.
https://www.irs.gov/instructions/i1041
NOTE : If the SS would have been non taxable on the decedent's return the it is non taxable on the 1041 which is why I said you will not report it since you only got one payment in January.
