It hath been ordained that every believer in God . . . shall, each day . . . repeat
“Alláh-u-Abhá” is an Arabic phrase meaning "God the All-Glorious". It is a form of the Greatest Name of God (see note 137). In Islám there is a tradition that among the many names of God, one was the greatest; however, the identity of this Greatest Name was hidden. Bahá’u’lláh has confirmed that the Greatest Name is "Bahá".
The various derivatives of the word "Bahá" are also regarded as the Greatest Name. Shoghi Effendi’s secretary writing on his behalf explains that

The Greatest Name is the Name of Bahá’u’lláh. "Yá Bahá'u'l-Abhá" is an invocation meaning: "O Thou Glory of Glories!". “Alláh-u-Abhá” is a greeting which means: "God the All-Glorious". Both refer to Bahá’u’lláh. By Greatest Name is meant that Bahá’u’lláh has appeared in God’s Greatest Name, in other words, that He is the supreme Manifestation of God.

The greeting “Alláh-u-Abhá” was adopted during the period of Bahá’u’lláh’s exile in Adrianople.
The repetition of “Alláh-u-Abhá” ninety-five times is to be preceded by the performance of ablutions (see note 34).