God's power is infinite. Whatever He desires to do, He does it, and whatever He does not desire to do, He does not do it. Nothing falls out of the premises of God's power and command. Without doubt, God's creation of the universe in six days or six stages does not in any way negate the infiniteness of His power. Never does it imply weakness or deficiency on the part of God, the Exalted. In other words, it is not impossible for God to create the world in an instant. However, the universe operates on cause-and-effect system and God does not want to make things run against the rules which pervade and permeate the entire universe. It is for the same underlying reason that God created the world in six days. In fact, we know that this gradual creation is owing to the characteristic of the universe itself not because He was not able to create it instantaneously.
The Holy Qur'an says:
هو الّذى خلق السّموات و الارض فى ستّة أیّامٍ
"And He it is Who created the heavens and the earth in six Days."
Firstly, it should be noted that the word "day" used in the Quranic verses refers to a period of time, not to its lexical and customary meaning. That is because, the word "day" according to the people of the earth is defined as the span of time it takes for the Earth or a celestial body (such as an other planet or a moon) to make a single rotation with respect to a star, measured most accurately from local noon to local noon. The word day used to signify a period of time (not 24 hours) is very common. What is in fact said by the Quran and traditions indicate that the earth has been created in six periods. No more details have been provided by the Quran as to the details of each period. What we deduce and conclude from the Quran is that the creation of the heaven and the earth has not taken place in an instant. The universe did not come into being in its present shape from nothingness; rather it came into being from something else which existed before it, i.e. a matter or substances of accumulated similar parts which God, the Almighty, separated from each other. Part of it was used to make the earth in two periods. Thereupon, the sky which was 'vapor' (or smoke) then was divided into parts and was made into seven firmaments.
Additionally, all the living creatures which we see around us are made of water. Hence, water (not the liquid which we call water) is the matter on which every living creature subsists.
Having said that, the meaning of the verse which is under discussion has now become clear; hence, the verse "And He it is Who created the heavens and the earth in six Days" means that the parts were first gathered, then separated from similar accumulated ingredients.
As of this point, the meaning of "the creation of the world in six days" has been made known to you. Now we shall explain the meaning of "Be and it is" in another verse. What does "Be and it is" mean? The Quran says:
إِنَّمَا أَمْرُهُ إِذَا أَرَادَ شیْئاً أَن یَقُولَ لَهُ کُن فَیَکُونُ
[His command, when He intends anything, is only to say to it: Be, so it is.]
This verse of the Holy Qur'an is one of the most brilliant verses which describe the word "creation" in relation to God. It says that God, the Exalted, does not need anything to create something and there is nothing to hinder God from creating. The word "command" in the above verse has been used in a metaphorical meaning. It is therefore not the antonym of "prohibit". The word indicates God's exalted status when He intends to create something. When He intends to create something, He uses the word "command". The sentence"اذا اردناه" means that when something becomes the subject of God's will, God's status is to tell that thing to "be" and that thing comes into existence. (Of course, it should be noted that it is not the word "be" that is used at the time of creation. In fact, it is only a matter of God's will. Also, there is no listener or addressee to listen to God's command with its two ears so as to come into existence. That is because, if there were something to listen to God's command, then it does not need to come into existence. It is already existent. Therefore, the verse under discussion has been used in an allegorical sense in the sense that whenever God intends anything, that thing comes into existence in an instant without any interruption or delay.)
In addition, what is granted or created by God does not accept any pause nor does it accept any change or gradualness. Whatever gradualness or delay we see in the beings is on their part not on the part of God, the Exalted.
Safvan bin Yahya says: "I asked Abil Hasan, Imam Reza (peace be upon him) to say something about God and the way He creates things. He said: Will in relation to us means the internal resolve [one's specific purpose in doing so, the end or goal that is aimed at, or intended to accomplish] or the faculty by which a person decides on and initiates action. When it comes to God, will means creating and initiating action not anything else because God does not need to think before He initiates an action. Such attributes do not apply to God; they are characteristic of human beings."
Therefore, the will of God is His very action not something else. It is the action to which God says "Be" and that action becomes existent. As we mentioned earlier God's saying or speech does not involve oral pronunciation of some words as He does not need to think and then decide. Since the essence of God has no quality, His actions are also free of any and all qualities.
Conclusively, if the Qur'an says that God created the universe in six days, it does not mean that God's action is gradual and that the creation took place in a gradual way, nor does it mean that God could not create the universe at once and as a whole because He needed time to pass. Such an assumption in regards to God's essence is completely wrong. In fact, gradualness is an essential quality of the creature not of Creator. As we said, whenever God commands something and says "be", that thing will become existent at that very instant. There is no time gap between God's command (or His will) and creation. He could create the world at once. However, the universe operates on cause-and-effect system and God does not want to make things run against the rules which pervade and permeate the entire universe. It is for the same underlying reason that God created the universe in six days or six periods. In fact, we know that this gradual creation is owing to the essential characteristic of the universe itself not because God was not able to create it. What is important to note is that God's power applies to anything which is intrinsically and ontologically possible.
 - Hud: 7
 - As stated by the commentary books, the word "smoke" or "vapor" does not refer to the so-called smoke or vapor.
 - Translation of Tafsir al-Mizan, vol.10, pg.224.
 - Translation of Tafsir al-Mizan, vol.10, pg.171. Under verse 82 of chapter Yasin.
 - Cited from the translation of Tafsir Al-Mizan, vol.17, pg.177
 - "ابى اللّه ان یجرى الاشیاء الّا باسباب" Cited from Collection of Works by Ayatollah Mutahhari, vol.1, pg. 408 (Majma'ul Bahrain, the word 'Sabab').
 - Collection of Works by Martyr Mutahhari, vol.4, pg. 626 and 627.