How to use a WordPress' existing admin icon?

Question

I use the add_menu_page function to add an new admin menu:

add_menu_page(
    'Custom_menu',
    'Custom_menu',
    'edit_posts',
    'custom_slug',
    '',
    'wordpress_existing_icon',
    5
);

How to use one of WordPress' existing icons?

For instance, if I would like to use the "Posts" WordPress icon, by what must I replace 'wordpress_existing_icon' in the code above ?

I tried 'edit' and 'edit-posts' but it doesn't work.