Tawheed is literally defined as absolute belief in Oneness of Allah. It validates a muslim to be a true believer while inability to practice it accordingly may lead to Shirk (disbelief). Practising it is rewarding while refusal to practice attracts punishment of Allah.
In the holy Qur’an, Allah said:
﴿خَلَقَ السَّمَـوَتِ بِغَيْرِ عَمَدٍ تَرَوْنَهَا وَأَلْقَى فِى الاٌّرْضِ رَوَاسِىَ أَن تَمِيدَ بِكُمْ وَبَثَّ فِيهَا مِن كُلِّ دَآبَّةٍ وَأَنزَلْنَا مِنَ السَّمَآءِ مَآءً فَأَنْبَتْنَا فِيهَا مِن كُلِّ زَوْجٍ كَرِيمٍ – هَـذَا خَلْقُ اللَّهِ فَأَرُونِى مَاذَا خَلَقَ الَّذِينَ مِن دُونِهِ بَلِ الظَّـلِمُونَ فِى ضَلَـلٍ مُّبِينٍ ﴾
(10. He has created the heavens without any pillars that you see, and has set on the earth firm mountains lest it should shake with you. And He has scattered therein moving creatures of all kinds. And We send down water from the sky, and We cause (plants) of every goodly kind to grow therein in pairs,) (11. This is the creation of Allah. So, show Me that which those besides Him have created. Nay, the wrongdoers are in plain error.)
Thus Allah explains His mighty power in creating the heavens and the earth, and everything that is within them and between them. He says:
﴿خَلَقَ السَّمَـوَتِ بِغَيْرِ عَمَدٍ﴾
(He has created the heavens without any pillars) Al-Hasan and Qatadah said, “It does not have any pillars, visible or invisible.”
﴿وَأَلْقَى فِى الاٌّرْضِ رَوَاسِىَ﴾
(and has set on the earth firm mountains) means, the mountains which stabilize and lend weight to the earth, lest it should shake with its water. Allah says:
﴿أَن تَمِيدَ بِكُمْ﴾
(lest it should shake with you.)
﴿وَبَثَّ فِيهَا مِن كُلِّ دَآبَّةٍ﴾
(And He has scattered therein moving creatures of all kinds) means, He has placed throughout it all kinds of animals, the total number of whose kinds and colors is known to no one except the One Who created them. When Allah tells us that He is the Creator, He also reminds us that He is the Provider, as He says:
﴿وَأَنزَلْنَا مِنَ السَّمَآءِ مَآءً فَأَنْبَتْنَا فِيهَا مِن كُلِّ زَوْجٍ كَرِيمٍ﴾
(And We send down water from the sky, and We cause (plants) of every goodly kind to grow therein in pairs,) meaning, every kind of good produce in pairs, i.e., they are beautiful to look at. Ash-Sha`bi said: “People are also produce of the earth, so whoever enters Paradise is goodly and whoever enters Hell is vile.”
﴿هَـذَا خَلْقُ اللَّهِ﴾
(This is the creation of Allah.) means, all that Allah has mentioned here of the creation of the heavens and earth and everything in between stems from His power of creation and control alone, and He has no partner or associate in that, Allah says:
﴿فَأَرُونِى مَاذَا خَلَقَ الَّذِينَ مِن دُونِهِ﴾
(So, show Me that which those besides Him have created.) those idols and rivals whom you worship and call upon.
﴿بَلِ الظَّـلِمُونَ﴾
(Nay, the wrongdoers) means the idolators who associate others in worship with Allah
﴿فِى ضَلَـلٍ﴾
(in error) means, they are ignorant and blind.
﴿مُّبِينٌ﴾
(plain) means, it is clear and obvious, and not at all hidden.
