The concept of God (Allah) in Islam is enunciated by God Himself
    in His revelation, the Holy Qur'an. It is expanded upon by the Final
    Prophet of God, Muhammad [peace upon him and his household]
    and further clarified by the successor to the Prophet, the Imam Ali.

        The Holy Qur'an

        The Unity

        In the name of Allah, the Beneficent, the Merciful.
                    Say: He, Allah, is One.
                Allah is He on Whom all depend.
               He begets not, nor is He begotten.
                     And none is like Him.

    [Source: The Holy Qur'an, translated by M. H. Shakir, Chapter
    112, verses 1-4]

    "Praise belongs to God, who in His firstness was solitary and in His
    beginninglessness was tremendously exalted through divinity and
    supremely great through His magnificence and power. He
    originated that which He produced and brought into being that
    which He created without a model preceding anything that He
    created. Our Lord, the eternal, unstitched (the heavens and the
    earth) through the subtlety of His lordship and the knowledge within
    His omniscience, created all that He created through the laws of
    His power, and split (the sky) through the light of dawn. So none
    changes His creation, none alters His handiwork, 'none repels His
    law' (XIII, 45) , none rejects His command. There is no place of
    rest away from His call, no cessation to His dominion and no
    interruption of His term. He is the truly existent from the first and
    the truly enduring forever. He is veiled from His creatures by His
    light in the high horizon, in the towering might, and in the lofty
    dominion. He is above all things and below all things. So He
    manifested Himself to His creation without being seen, and He
    transcends being gazed upon. He wanted to be distinguished by the
    profession of Unity when He withdrew behind the veil of His light,
    rose high in His exaltation and concealed Himself from His creation.

    "He sent to them messengers so they might be His conclusive
    argument against His creatures and so His messengers to them
    might be witnesses against them. He sent among them prophets
    bearing good tidings and warning, 'that whosoever perished might
    perish by a clear sign, and by a clear sign he might live who lived'
    (VIII, 42) and that the servants might understand of their Lord that
    of which they had been ignorant, recognise Him in His Lordship
    after they had denied (it) and profess His Unity in His divinity after
    they had stubbornly resisted."

    [Source: Muhammad Baqir Majlisi, Bihar al-Anwar, vol. 4, pp.
    287-8]

     "Praise belongs to God, who did not originate from anything, nor
    did He bring what exists into being from anything. His
    beginninglessness is attested to by the temporality of things, His
    power by the impotence with which He has branded them, and His
    everlastingness by the annihilation which He has forced upon them.
    No place is empty of Him that He might be perceived through
    localization, no object is like Him that He might be described by
    quality, nor is He absent from anything that He might be known
    through situation.

    "He is distinct in attributes from all that He has originated,
    inaccessable to perception because of the changing essences He
    has created (in things), and outside of all domination by changing
    states because of grandeur and tremendousness. Forbidden is His
    delimitation to the penetrating acumen of sagacities, His description
    to the piercing profundities of thought and His representation to the
    searching probes of insight.

    "Because of His tremendousness places encompass Him not,
    because of His majesty measures gauge Him not, and because of
    His grandeur standards judge Him not. Impossible is it for
    imaginations to fathom Him, understandings to comprehend Him or
    minds to imagine Him. Powers of reason with lofty aspiration
    despair of contriving to comprehend Him, oceans of knowledge run
    dry without alluding to Him in depth , and the subtleties of
    disputants fall from loftiness to pettiness in describing His power.

    "He is One, not in terms of number; Everlasting, without duration;
    Standing, without supports. He is not of a kind that other kinds
    should be on a par with Him, nor an object that objects should be
    similar to Him, nor

    like things that attributes should apply to Him. Powers of reason go
    astray in the waves of the current of perceiving Him, imaginations
    are bewildered at encompassing the mention of His
    beginninglessness, understandings are held back from becoming
    conscious of the description of His power, and minds are drowned
    in the depths of the heavens of His kingdom.

    "He is Master over (giving) bounties, Inaccessable through
    Grandeur, and Sovereign over all things. Time makes Him not old,
    nor does description encompass Him. Humbled before Him are the
    firmest of obduracies in the limits of their constancy, and submitted
    to Him are the most unshakeable of the cords in the extremity of
    their towering regions .

    "Witness to His Lordship is the totality of kinds, to His Power their
    incapacity, to His eternity their createdness, and to His permanence
    their passing into extinction. So they possess no place of refuge
    from His grasp of them, no exit from His encompassing them, no
    way of veiling themselves from His enumeration of them and no
    way of avoiding His power over them. Sufficient is the perfection of
    His making them as a sign, His compounding of their (natural)
    constitutions as a proof, the temporal origin of their natures as (a
    reason for His) eternity, and the creation's laws governing them as a
    lesson . No limit is attributed to Him, no similitude struck for Him
    and nothing veiled from Him. High indeed is He exalted above the
    striking of similitudes and above created attributes!

    "And I testify that there is no god but He, having faith in His
    Lordship and opposing whoso denies Him; and I testify that
    Muhammad (PBUH) is His servant and messenger, residing in the
    best lodging-place, having passed from the noblest of loins and
    immaculate wombs, extracted in lineage from the noblest of mines
    and in origin from the most excellent of plantations, and (derived)
    from the most inaccessible of summits and the most glorious roots,
    from the tree from which God fashioned His prophets and chose
    His trusted ones: (a tree) of excellent wood, harmonious stature,
    lofty branches, flourishing limbs, ripened fruit, (and) noble interior,
    implanted in generosity and cultivated in a sacred precinct. There it
    put forth branches and fruit, became strong and unassailable, and
    then made him (the prophet Muhammad PBUH) tall and eminent,
    until God, the Mighty and Majestic, honoured him with the Faithful
    Spirit , the Illuminating Light , and the Manifest Book . He
    subjected to him Buraq and the angels greeted him. By means of
    him He terrified the devils, overthrew the idols and the gods (who
    were) worshipped apart from Him. His prophet's Wont (sunnah)is
    integrity, his conduct is justice and his decision is truth. He
    proclaimed that which was commanded by his Lord , and he
    delivered that with which he was charged until he made plain his
    mission through the profession of Unity and made manifest among
    the creatures that there is no god but God alone and that He has no
    associate; until His Oneness became pure and His lordship
    unmixed. God made manifest his argument through the profession
    of His Unity and He raised his degree with submission (al-islam).
    And God, the Mighty and Majestic, chose for His prophet what
    was with Him of repose, degree and means - upon him and upon
    his pure household be God's peace."
   [Source: Muhammad Baqir Majlisi, Bihar al-Anwar, vol. 4, pp.
    221-3]