OnCollisionEnter inside FixedUpdate?

I'd like to add this code to a script, but I'm not sure it's correct.

function OnCollisionEnter (collision : Collision) 
    print (collision.collider);
    collision.rigidbody.AddForce (Vector3.up * 1000);

Since it includes AddForce, should it be inside FixedUpdate? But then, I guess I can't include the whole OnCollisionEnter function inside FixedUpdate?

OnCollisionEnter is added at the root of your class like any other function.