Julia ode: Jacobi function does not get called

Hello, I have a problem with the following example. It just represents he problem from another code i wrote. I want to use the function f_jac to supply the jacobian to the ode solver. But somehow the function does not get called. And changes in the jacobi function have no impact on the solution.
The code:

function f(du,u,p,t)
    du[1] = 2.0 * u[1] - 1.2 * u[1]*u[2]
    du[2] = -3 * u[2] + u[1]*u[2]

  function f_jac(J,u,p,t)
    J[1,1] = 2.0 - 1.2 * u[2]
    J[1,2] = -1.2 * u[1]
    J[2,1] = 1 * u[2]
    J[2,2] = -3 + u[1] 

  ff = ODEFunction(f,jac = f_jac)

  prob = ODEProblem(ff,ones(2),(0.0,10.0))

  Solution = solve(prob)

  p = plot(Solution)

That’s a non-stiff ODE, so it probably just didn’t need to use it.

Thank you. Sometimes the solution is really simple^^