Custom content before post content

You can check the post type of the current post via get_post_type:

function theme_slug_filter_the_content( $content ) {
    if( 'post' == get_post_type() ){
        $custom_content="YOUR CONTENT GOES HERE";
        return $custom_content . $content;
    }
    return $content;
}
add_filter( 'the_content', 'theme_slug_filter_the_content' );