What is the equivalence to the free field in the interaction picture?

[naima]

I read in this paper[/PLAIN]
"Unfortunately, the derivation of the Dyson series perturbation expansion relies on the use of the interaction picture, in which the interacting field is unitarily equivalent to the free field"

It may be obvious but could you tell me where this equivalence to the free field appears in
interaction picture

 

[naima]

When you get no answer the best is to open textbooks. It is what i did.
Open your Sredicki p70 ex 9.5)