Hi BvU, thanks for your reply. The equation comes from the textbook "University of Physics - Modern physics" by Zemansky (https://drive.google.com/drive/folders/13rAnc_ryXgKa3YQOXms2AGnbpFvqeXXd , book page 1268)
Yes its the chapter "38.4 WAVE–PARTICLE DUALITY, PROBABILITY, AND UNCERTAINTY"...