In several places, the Quran implies that it was sent down all at once on a specific night.
Ramadhan is the (month) in which was sent down (onzila) the Qur'an, as a guide to mankind, also clear (Signs) for guidance and judgment (Between right and wrong). So every one of you who is present (at his home) during that month should spend it in fasting, but if any one is ill, or on a journey, the prescribed period (Should be made up) by days later. God intends every facility for you; He does not want to put to difficulties. (He wants you) to complete the prescribed period, and to glorify Him in that He has guided you; and perchance ye shall be grateful. S. 2:185 Y. Ali
Ha-Mim. By the manifest Book (this Qur'an) that makes things clear, We sent it (this Qur'an) down (anzalnahu) on a blessed night [(i.e. night of Qadr, Surah No: 97) in the month of Ramadan, the 9th month of the Islamic calendar]. Verily, We are ever warning [mankind that Our Torment will reach those who disbelieve in Our Oneness of Lordship and in Our Oneness of worship]. Therein (that night) is decreed every matter of ordainments. Amran (i.e. a Command or this Qur'an or the Decree of every matter) from Us. Verily, We are ever sending (the Messengers), (As) a Mercy from your Lord. Verily! He is the All-Hearer, the All-Knower. The Lord of the heavens and the earth and all that is between them, if you (but) have a faith with certainty. S. 44:1-7 Hilali-Khan
Behold, We sent IT down (anzalnahu) on the Night of Power; And what shall teach thee what is the Night of Power? The Night of Power is better than a thousand months; in it the angels and the Spirit descend, by the leave of their Lord, upon every command. Peace it is, till the rising of dawn. S. 97:1-5 Arberry
The above passages presuppose that the entirety of the Muslim scripture was "revealed" all at once. Note that the texts do not speak of parts of the Quran being sent down but that it itself was given on a specific night in the month of Ramadan. Lest we be accused of distorting the meaning of the afore quoted references note how the following commentary explains Q. 97:1:
Lo! We revealed it, that is, the Quran, IN ITS ENTIRETY, [sending it down] from the Preserved Tablet to the heaven of this world, on the Night of Ordainment, that is, [the Night] of great eminence. (Tafsir al-Jalalayn; source; capital emphasis ours)
Yet these Quranic passages contradict the following texts:
We sent down (anzalnahu) the (Qur'an) in Truth, and in Truth has it descended (nazala): and We sent thee but to give Glad Tidings and to warn (sinners). (It is) a Qur'an which We have divided (into parts from time to time), in order that thou mightest recite it to men at intervals: We have revealed it (wa-nazzalnahu) by stages. S. 17:105-106 Y. Ali
And those who disbelieve say: "Why is not the Qur'an revealed (nuzzila) to him all at once?" Thus (it is sent down in parts), that We may strengthen your heart thereby. And We have revealed it to you gradually, in stages. (It was revealed to the Prophet in 23 years.). S. 25:32 Hilali-Khan
These conflicting and contradictory statements have caused Muslim scholars to come up with a convenient ad hoc explanation. It is asserted that the entire Quran was sent down from Allah himself to the lowest heaven where it remained. From there Allah sent Jibril/Gabriel to reveal portions of the book to Muhammad over a period of twenty-three years!
As renowned Salafi scholar Dr. Abu Ameenah Bilal Philips explains:
According to Allaahs statements in His Book, there were two distinct revelations of the Quraan which took place. It is important that these two revelations be understood in order to clear up the apparent contradictions [sic] in the various terms used in the Quraan and Sunnah to describe the Quraans revelation. On one hand, the Quraan is referred to as having been revealed in its totality in Ramadaan or on Laylatul-Qadr, the Night of Decree; while on the other hand, it is referred to as having been continuously revealed in segments up until the death of the Prophet.
The First Revelation
Allaah caused the Quraan to descend from the Protected Tablet (al-Lawh al-Mahfooth) on which it was written to the lowest heaven. In this revelation all of the Quraan was sent down at one time to a station in the lowest heaven referred to as "Bayt al-Izzah" (The House of Honor or Power). The blessed night on which this descent took place is called "Laylatul-Qadr" (The Night of Decree), one of the odd-numbered nights in the last ten days of the month of Ramadaan. Allaah referred to this initial revelation as follows [quotes Q. 44:1-3; 97:1; 2:185]
These verses have [sic] to refer to the initial revelation because it is a known fact [sic] that the whole Quraan was not revealed to Prophet Muhammad on a single night on Ramadaan.
Ibn Abbaas stated that the Quraan was first separated from its station in the upper heavens and placed in Bayt al-Izzah in the lowest heaven. One version states that this took place on the Night of Decree in Ramadaan. Had it been Allaahs wish, the Quraan could then have been revealed as a whole to the Prophet in a single revelation. This was the method by which all of the earlier books of revelation were sent down. But, Allah chose to divide the revelation into two parts [sic]. The first revelation within the heavens represented an announcement to the inhabitants of the heavens that the final book of revelation was being sent down upon the last of the prophets. (Philips, Usool at-Tafseer, 6. The Revelations of the Quraan, pp. 94-96; sources 1, 2; comments within brackets and underline emphasis ours)
Another Islamic source says:
(The month of Ramadan is) the month in (which was revealed the Qur'an) whereupon Gabriel brought down the ENTIRE Qur'an to the first heaven, dictated it to the scribes among the angels (al-safarah) and then took it down to Muhammad (pbuh) day after day, sometimes revealing to him just one, two or three verses and sometimes an entire surah, (Tanwîr al-Miqbâs min Tafsîr Ibn Abbâs; source; capital and underline emphasis ours)
And from his narration on the authority of Ibn 'Abbas that he said regarding the interpretation of Allah's saying (Lo! We revealed it): '(Lo! We revealed it) He says: We sent Gabriel with the entire Qur'an to the scribes of the nether heaven (on the Night of Power) on the night of judgement and decree; it is also said this means: on a night blessed with forgiveness and mercy, and then it was revealed to the Prophet (pbuh) in instalments. (Tanwîr al-Miqbâs min Tafsîr Ibn Abbâs; source)
The reader can immediately spot the problem with the above assertion. The Quran nowhere says that it was revealed in two stages, one in which the entire revelation was sent down at once to the lowest heaven, and the other where only parts of it was conveyed to Muhammad. This is simply a desperate attempt of explaining away the gross error within the Muslim scripture which claims in certain places that Allah sent down the Quran as a whole on a certain night, thereby contradicting those passages that state that Muhammad received the so-called "revelations" in segments.
In fact, one of the verses expressly says that the Quran was sent down in the month of Ramadan as a guide to mankind:
Ramadhan is the (month) in which was sent down the Qur'an, as a guide to mankind, S. 2:185
Instead of saying that the Quran was given to the angels in the lowest heaven in Ramadan, the passage actually claims that the book was sent down for the guidance of mankind. This certainly sounds like it was sent down to mankind in Ramadan, in seeming contradiction to the theory postulated by the above scholars.
There are some passages in the Quran that have an indirect bearing on this issue. Although they do not explicitly speak about the descent of the Quran, they do not make much sense if we assume the scenario that the Quran was sent down piece by piece over a 23-year time period. Therefore, they seem to support the understanding that the Quran was given all at once. See the discussion in the article, Is half the Quran already fully detailed?
On the other hand, there are also passages in the Quran which are difficult to reconcile with the claim that the Quran was given as one complete book, see the discussion in Examining the inherent problems with the descent of the Quran.
Sam Shamoun answers to one Muslim apologist who responded to the above article: Revisiting the issue of the Qurans "Descent" All at once or one piece at a time?
Contradictions in the Qur'an
Answering Islam Home Page