What is the divergence of an identity matrix?

I want to take the divergence of a 3x3 identity matrix. What will this turn out to be? I'm deriving the Navier-Stokes equations from first principles, and this part has me stuck...

I have to take the following divergence: div(-pI), where p is a scalar (pressure) and I is the 3x3 identity matrix. It should turn out to be the negative gradient of pressure -grad(p), but I don't know anything about the intermediate steps. Any help would be appreciated.

Thanks!