This is contained in Michors book "Manifolds of differentiable mappings" (available online here [1]). It is a bit hidden (see p. 35 under point 4., the relevant result for scalar multiplication is 4.4.3 earlier), since Michor wants to discuss suitable topologies on all smooth maps not only on the compactly supported functions. 
Michor constructs the Whitney topology via jet bundles.


[1]: http://www.mat.univie.ac.at/~michor/manifolds_of_differentiable_mappings.pdf