He is the One Who is singled out in all aspects of Perfection such that nothing else shares with Him in them. It is obligatory upon the servants to single Him out alone in belief, saying and action by acknowledging His unrestricted perfection, His uniqueness and singling Him out Alone for all types of worship.
"'O two companions of the prison! Are many different lords better or Allah, the One, the Irresistible?'" (12:39)
"Say: I am only a warner and there is no god except Allah the One, the Irresistible." (38:65)
The meaning of al-Ahad is the Unique, who is forever One and has never and will never have another alongside Him. (al-Nihaayah, 1/35).
Allaah is Unique in His Oneness, in being One in His essence and attributes. Some (scholars) differentiated between al-Waahid and al-Ahad by saying that al-Waahid refers to His being one in His essence only, whilst al-Ahad means that He is one in both His essence and His attributes. (Tafseer Asmaa’ Allaah by al-Zajjaaj, p. 58).
And it was said that al-Waahid means He is unique in His essence and does not have any peer or rival, and al-Ahad is Unique in His attributes. So al-Waahid is one in and of Himself, and al-Ahad means being one in His attributes. Al-Ahad is one of the attributes of Allaah which belong uniquely to Him, and in which nothing else has a share. (Lisaan al-‘Arab –Ahad – 1/35; wahida 8/4779 – 4783). Allaah is al-Waahid al-Ahad, and has no second, no partner, no peer and no rival. He is al-Waahid upon Whom His slaves depend and Whom they seek. They do not put their trust in anyone except Him. (Ishtiqaaq Asmaa’ Allaah, 90-93). He is al-Waahid and there is none like unto Him. Everything other than Him that is called one in some aspect will not be one in many other aspects. (Sha’n al-Du’aa’, 82-83)
Ibn Jareer believed that the Oneness of Allaah ruled out there being anything else that could be like Him. Allaah has no peer and no rival. He is the One God, the One Lord, and no one other than Him deserves to be obeyed; no one other than Him should beworshipped. (Tafseer Ibn Jareer, 3/265-266)
And Allaah says (interpretation of the meaning):
“And your Ilaah (God) is One Ilaah (God — Allaah)” [al-Baqarah 2:163]
Allaah is One and is Perfect in all ways; He has no partner or associate. His slaves must affirm His Oneness in their hearts and in their words and deeds, by acknowledging His absolute perfection and directing all acts of worship solely to Him. (Tayseer al-Kareem al-Rahmaan, 5/485)