Unfortunately it has to be reported in the year sold. However, if this was your personal home, amending the 2015 return may not make any difference, since you can't claim a personal loss, and most "main home" gain is excludable. I would suggest you print out a copy of your 2015 return, and then do an amendment where you delete the sale, to see what the result is. If the final number doesn't change (i.e. you don't owe any tax or have more refund coming) then you don't need to amend.
If this was business property (rental or other) then you definitely need to amend to take the sale off 2015 and report it for 2016.