Bumps on tree
The leaves on your linden have spindle galls caused by feeding of eriophyid mites from the undersides of the leaves. Eriophyid mites are very small and slightly elongated; they are a much different kind of mites than the spider mites you may be familiar with.
The leaf galls won’t damage the tree’s health or well-being. No sprays or other treatments are needed.
If you prefer to try treatment, consider hiring a Certified Arborist to spray the tree in spring, at bud break. You can locate local Certified Arborists by using your zip code at https://www.treesaregood.org/findanarborist/findanarborist.