The guardian is assured the guidance of both Bahá'u'lláh and the Bab

"Instructions sent on behalf of the Guardian are binding, as are the words of the Guardian; although of course, they are not the Guardian's words.

"The Guardian's infallibility covers interpretation of the revealed word and its application. Likewise any instructions he may issue having to do with the protection of the Faith, or its well-being must be closely obeyed, as he is infallible in the protection of the Faith. He is assured the guidance of both Bahá'u'lláh and the Bab, as the Will and Testament of Abdu'l-Bahá clearly reveals."

(From a letter written on behalf of the Guardian to an individual believer, August 20, 1956)

http://bahai-library.com/hornby_lights_guidance_2.html&chapter=2#n1055