QGIS - Least Cost Analysis - Removing waterbodies

I’m new to least cost analysis. I have a DEM-layer of a region, an I also have created slope layer. The purpose is to calculate the least cost path between to ancient sites with QGIS. However, in the region there are lakes and river (I have the Shapefiles), and the problem is that the resulted least cost path crosses through the lakes and rivers and I don’t want that. I want them to be constraints, so the paths should find another way to the destination point.

How do I make the least cost tool take them into consideration?