pre_get_comments or the_content filter

As noted by @cybmeta the most convenient way to add something on every post would be to use the_content filter.

This may give you unexpected results on archive pages, because some themes use the_content to display information on those, while others use the_excerpt. So, depending on what you want, you may want to use the filter on the_content only on singular pages or add the same filter to the excerpt as well.`

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