First—Muhammad, peace and blessings of Allah be upon him, was in doubt about his own prophethood and about the Holy Quran being the Word of Allah, as is written in Surah al-Baqarah and Surah al-An‘am:
This proves that in his heart Muhammad, peace and blessings of Allah be upon him, knew with conviction that he was not a Messenger of God. If he had been a Messenger of God, or if he had ever shown any miracle, or had the Mi‘raj [Spiritual Ascent to Heaven] taken place, or if Gabriel, peace be upon him, had brought the Holy Quran, he would never have been in doubt about his own prophethood. This clearly proves that he was in doubt about the Holy Quran and about his own prophethood. Hence, he is not a Messenger of Allah.
Second—If Muhammad, peace and blessings of Allah be upon him, had been a Messenger, he would not have responded so helplessly to questions put to him at that time, saying that God knows; meaning, I do not know. Nor would he have spoken incorrectly concerning the number of Ashab-e-Kahf [the People of the Cave]. Likewise, he would not have said that the Sun becomes concealed in, or sinks into, a murky pool. As a matter of fact, the Sun is 90 million times larger than the earth, so how can it possibly become concealed in a pool of murky water?
Third—Muhammad, peace and blessings of Allah be upon him, was never ever granted any miracle, as is stated in Surah al-‘Ankabut (Translation of the Arabic): ‘They say, “Why were not some Signs sent down for him from his Lord”.’ (This means not even one, because the word —la in this verse, which is categorical, implies the negation of the class as a whole.)
Then, in Surah Bani Isra’il, we also read: ‘And We ceased to send Signs because people had rejected them before.’ By this, it is clearly evident that God did not give him any miracle. In reality, had he been given even one miracle, he would not have been in doubt about his prophethood or the Quran.
1 Surah al-Baqarah, 2:148; Surah al-An‘am, 6:115 [Publisher]