What is the proper method to give credits in lecture material
Do you include the slides verbatim (e.g., by copying a pdf or image into your slides), or do you just base your slides on those other slides in the same way in which you might base them on a textbook?

In the first case, I think that a note on that slide (on the first slide, if you copy several slides in a row) is mandatory – probably not only a reference, but a copyright notice.

In the second case, I'd include the reference in a bibliography section, like for other journal papers or textbooks that you base your slides on. If I can avoid it, I prefer not to clutter every slide with details that are irrelevant for understanding the content.