On the subject of Oneness of Allah / Tawheed Imam Reza(as) said:

"The first step toward worship of Allah is to know Him, and the root of that knowledge is in recognizing His Oneness; And the principle of that recognition lies in negating all definitions from Him;

Every reason guides to the fact that anything defined is created (and not the creator), And every created one testifies that it has a creator, who is not created.

The one, who is not contingent, is the One, timeless in eternity. So he who ascribes attributes to Him has not (truly) worshipped Him, And he who knows Him by similitude has not understood His oneness; And one who compares Him (to anything) has not understood the true essence, nor has he confirmed Him if he sets any limit for Him; And he who points towards Him with any of his senses is, in fact, aiming at something other than Him;

So, he who likens Him (to others) does not actually mean Him, and he who considers Him a part of the whole, does not actually know Him, and he who imagines Him is indeed deluded.

Everything, which can be encompassed by knowledge and imagination, is created, and everything that is dependent upon others for its existence is an effect.

By His creation, we are guided to His existence, and by reason we believe in (the necessity of) knowing Him, and by the innate nature (of human beings) His Proof is established.

His creation is a veil between Him and His creatures, and He is distinct from them by His disparateness.

His beginning of their creation is the proof that He has no beginning, for the one who is begun (in time) cannot be a (timeless) beginner; So, all the names used for Him are mere interpretations, and acts attributed to Him are meant for common understanding.

Indeed, he who defines or sets a limit to Him, displays ignorance about Him, and he who tries to encompass Him transgresses the bounds. And he who makes a surmise of His Being, misses Him (altogether). Whoever says 'How is He?' has indeed invoked His likeness, and whoever asks 'Why?' has indeed indulged in explaining Him away. And he who says 'When?' sets a time limit for Him, and he who asks 'In what?' tries to contain Him. And whoever asks 'Where to?' confines Him, and whoever asks 'Till when?' indeed fixes (an imagined) goal for Him. Thus whoever fixes a goal for Him, limits Him, and to set a limit for Him is to disbelieve in Him.

The discrepancy among the creatures does not effect any variation in Him, and the limitation of the limited ones creates no confine for Him. He is One but not as number, and He is Manifest but not by was of associating; He is Evident but not by way of being visible. He is intrinsic but not by separation, and He is apart but not by distance. And He is near but not by proximity. He is (truly) subtle, but not in body, He Exists but not from nothingness. He is the doer but not under any duress, and He is the assessor but not yielded to pondering; He is designer without needing any movement. He Wills without being driven by resolves or determination, He perceives without depending on any senses. He hears and sees without any organ.

Time does not accompany Him, places do not contain Him, slumber does not overtake Him, attributes do not define Him, and implements do not benefit Him. His Existence precedes time, His Being precedes void and nothingness. His Eternity precedes all beginning.

By His creating similar things, it is established that He has no likeness, and by His creating dissimilarities among things, it is evident that He has no contrast. And by equation among things, it is understood that He has no equal.

He contrasted light with darkness, severe winter with hot wind. He puts together things which are divergent, and separates things which are closer to each other. Thus by separating, He guides us to its Separator and by putting things together, He guides to its Integrator. Allah, Most High, says (in Qur'an): 'We have created a pair, that perchance you might remember.'(51:49)

The meaning of Sustainer applied to Him when there was no one sustained, and the essence of (all) Divine Power belonged to Him when there was no worshipper, and He was the Knower when there was nothing knowable.

He did not merit the meaning of being Creator after having created, nor did He become the Originator after having originated.

He is not caused to be absent by 'since,' nor is He brought nearer by 'indeed.' And 'perchance' does not veil Him, nor is He limited in time by 'when.' He is not contained by 'then,' nor is He accompanied by 'with.'

Every effect that is seen in the creation does not exist in its Creator, and all that is contingent in it is not possible in its Maker. Movement and Stillness do not occur in Him. And how can they occur in Him Who caused them? How could that which He initiated return to be applied to Him? In that case, there would have risen discrepancy in His Being, and the meaning of Eternal would be inapplicable to Him, and the word Creator would have had its opposite meaning.

If He were bound by rear, He would have been bound by fore also, and if completeness were solicited for Him, then incompleteness would have been necessarily conceivable for Him. How could He be worthy of being called Eternal if He were contingent? How can He initiate things if He Himself needed to be initiated? In that case, the signs of being created would be evident in Him, and He would have changed from being the Indicator into the one indicated. Any statement contradicting this truth has no proof, nor do the questions arising therefrom have any answers.

There is no god but Allah, Most High and Most Mighty. (And may Allah bless His Prophet, Muhammad and his pure Progeny.)"

Source: Al Amali of Shaikh Mufeed, Chapter 30, Hadees 4