I believe it’s stated explicitly somewhere but can’t seem to find that at the moment. I’ll update when I do, but until then we only have it implicitly in the various locations (here, Saving Throws) where modifier calculations are found.
Will saving throws measure how well you can resist attacks to your mind and spirit. They use your Wisdom modifier and are calculated as shown in the formula below.
Will save result = d20 roll + Wisdom modifier + proficiency bonus + other bonuses + penalties
Sometimes you’ll need to know your DC for a given saving throw. The DC for a saving throw is 10 + the total modifier for that saving throw.
Note that the DC is ‘the total modifier’ added to 10, and the modifier calculation includes penalties.