Bahá ` u 'll áh 's Kitáb-i-Aqdas and The Will and Testament of ` Abdu ` l-Bahá are foundational documents of the Bahá ' í administrative order .
Bahá ` u 'll áh had written that ` Abdu ` l-Bahá had special powers , and that God would make sure that if ` Abdu ` l-Bahá explained anything that Bahá ` u 'll áh said , then the Bahá ' ís should believe that explanation .