BahÃ ` u 'll Ã h called him the `` Expounder '' of his word , meaning that only ` Abdu ` l-Bah Ã had the right to say what BahÃ ` u 'll Ã h 's writings mean . After BahÃ ` u 'll Ã h died on 29 May 1892 , the Will and Testament of BahÃ ` u 'll Ã h named ` Abdu ` l-Bah Ã as Centre of the Covenant , successor and interpreter of BahÃ ` u 'll Ã h 's writings .