Credits Shortcode plugin allows to add Source Link or Via Link for a post or a page. Shortcodes can be added directly or can be added using the button in the WordPress Editor window.
[credits link=»http://example.com» type=»source»] Source Name [/credits]
[credits link=»http://example.com» type=»via»] Via Name [/credits]
Te puedes descargar este plugin para utilizarlo en tu instalación autoalojada de WordPress.