Tauheed (Islamic Monotheism) has three aspects:
(A) Tauheed-ar-Ruboobiyyah (Oneness of the Lordship of Allah)
To believe that there is only one Lord for the entire universe and He is, its Creator, Organizer, Planner, Sustainer, and the Giver of security, and that is Allah.
“Allah is the Creator of all things, and He is the Guardian and Disposer of all affairs.” (Quran 39:62)
“No kind of calamity can occur, except by the leave of Allah: and if any one believes in Allah, (Allah) guides his heart (aright): for Allah knows all things.” (Quran 64:11)
(B) Tauheed-al-Uloohiyyah (Oneness of the worship of Allah)
To believe that none has the right to be worshipped [e.g. praying, invoking, asking for help (from the unseen), swearing, slaughtering sacrifices, giving charity, fasting, pilgrimage] but Allah.
"And your Lord says: “Call on Me; I will answer your (prayer): but those who are too arrogant to serve Me will surely enter Hell abased!” (Quran 40:60)
“You alone do we worship, and you alone do we ask for help” (Quran 01:04)
”Say: Truly, my prayer and my service of sacrifice, my life and my death, are (all) for Allah, the Cherisher of the Worlds” (Quran 6:162)
(C) Tauheed-al-Asmaa was-Sifaat (Oneness of the Names and Qualities of Allah)
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We must not name or qualify Allah except with what He or His Messenger (peace be upon him) have named or qualified Him.
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None can be named or qualified with the Names or Qualifications of Allah e.g. Al-Kareem.
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We must believe in all the Qualities of Allah which Allah has stated in His Book (the Quran) or mentioned through His Messenger Muhammad (peace be upon him) without changing their meaning, ignoring them completely, twisting the meanings or giving resemblance to any of the created things
“Allah. There is no god but He! To Him belong the most Beautiful Names.” (Quran 20:08)
“(He is) the Creator of the heavens and the earth… there is nothing whatever like unto Him, and He is the One that hears and sees.” (Quran 42:11)
These three aspects of Tauheed are included in the meaning of La ilaha ilallah (There is no god but Allah).