Should I report my rental income for how much I collected in 2016 or for the entire 12 month contract ?. Tenant owned me 2 months from 2016, so I shouldn' pay tax on that

I have a condo rented d did not collect all my rental income during 2016.  Should I report as income only the amounts collected or the entire 12 months contract ?. I mean if rental income is reported when accrued or when collected.  thanks