During his prophethood, Muhammad claimed that his revelations were delivered to him by the angel Gabriel. One source for Muhammad's first encounter with Gabriel is Sirat Rasul Allah ("The Life of the Prophet"), which is the earliest biography of Muhammad, written by Ibn Ishaq (704-767 AD) and Ibn Hisham (died 833 AD):
Sirat Rasul Allah, Guillaume, pg. 106-107
"He came to me," said the apostle of God, "while I was asleep, with a coverlet of brocade whereon was some writing, and said, "Read!" I said, "What shall I read?" He pressed me with it so tightly that I thought it was death; then he let me go and said, "Read!" I said, "What shall I read?" He pressed me with it again so that I thought it was death; then he let me go and said "Read!" I said, "What shall I read?" He pressed me with it the third time so that I thought it was death and said "Read!" I said, "What then shall I read?" - and this I said only to deliver myself from him, lest he should do the same to me again.
He said: "Read in the name of thy Lord who created, Who created man of blood coagulated. Read! Thy Lord is the most beneficent, Who taught by the pen, Taught that which they knew not unto men." So I read it, and he departed from me.
And I awoke from my sleep, and it was as though these words were written on my heart. Now none of God's creatures was more hateful to me than an (ecstatic) poet or a man possessed; I could not even look at them. I thought, Woe is me poet or possessed - Never shall Quraysh say this of me! I will go to the top of the mountain and throw myself down that I may kill myself and gain rest.
So I went forth to do so and then when I was midway on the mountain, I heard a voice from heaven saying, "O Muhammad! thou art the apostle of God, and I am Gabriel." I raised my head towards heaven to see who was speaking, and Gabriel in the form of a man with feet astride the horizon, saying, "O Muhammad! thou art the apostle of God and I am Gabriel." I stood gazing at him, and that turned me from my purpose moving neither forward nor backward; then I began to turn my face away from him but towards whatever region of the sky I looked, I saw him as before.
And I continued standing there, neither advancing nor turning back, until Khadija sent her messengers in search of me and they gained the high ground above Mecca and returned to her while I was standing in the same place; then he parted from me and I from him, returning to my family. And I came to Khadija and sat by her thigh and drew close to her. She said, "O Abul-Qasim, where hast thou been? By God, I sent my messengers in search of thee, and they reached the high ground above Mecca and returned to me."
I said to her, "Woe is me poet or possessed." She said, "I take refuge in God from that, O Abul-Qasim. God would not treat you thus since he knows your truthfulness, your great trustworthiness, your fine character, and your kindness. This cannot be, my dear. Perhaps you did see something." "Yes, I did," I said. Then I told her of what I had seen; and she said, "Rejoice, O son of my uncle, and be of good heart. Verily, by Him in whose hand is Khadija's soul, I have hope that thou wilt be the prophet of this people."
Then she rose and gathered her garments about her and set forth to her cousin Waraqa b. Naufal b. Asad b. Abdul-Uzza b. Qusayy, who had become a Christian and read the scriptures and learned from those that follow the Torah and the Gospel. And when she related to him what the apostle of God told her he had seen and heard Waraqa cried, "Holy! Holy! Verily by Him in whose hand is Waraqa's soul, If thou hast spoken to me the truth, O Khadija, there hath come unto him the greatest Namus (meaning Gabriel) who came to Moses aforetime, and, lo, he is the prophet of this people. Bid him be of good heart." So Khadija returned to the apostle of God and told him what Waraqa had said.
The account of Muhammad's first encounter with Gabriel is also found in Sahih al-Bukhari 3, 4953, 6982, Sahih Muslim 160a-c, and The History of al-Tabari (Volume 6, 1147).
Notice the following elements from the account:
When examining angelic encounters in Scripture, a pattern emerges wherein an angel appears, people are startled, and the angel immediately attempts to assuage their fear:
Matthew 28:2-5
2 And, behold, there was a great earthquake: for the angel of the Lord descended from heaven, and came and rolled back the stone from the door, and sat upon it.
3 His countenance was like lightning, and his raiment white as snow:
4 And for fear of him the keepers did shake, and became as dead men.
5 And the angel answered and said unto the women, Fear not ye: for I know that ye seek Jesus, which was crucified.
Luke 1:11-13, 19
11 And there appeared unto him an angel of the Lord standing on the right side of the altar of incense.
12 And when Zacharias saw him, he was troubled, and fear fell upon him.
13 But the angel said unto him, Fear not, Zacharias: for thy prayer is heard; and thy wife Elisabeth shall bear thee a son, and thou shalt call his name John.
19 And the angel answering said unto him, I am Gabriel, that stand in the presence of God; and am sent to speak unto thee, and to shew thee these glad tidings.
Luke 1:26-30
26 And in the sixth month the angel Gabriel was sent from God unto a city of Galilee, named Nazareth,
27 To a virgin espoused to a man whose name was Joseph, of the house of David; and the virgin's name was Mary.
28 And the angel came in unto her, and said, Hail, thou that art highly favoured, the Lord is with thee: blessed art thou among women.
29 And when she saw him, she was troubled at his saying, and cast in her mind what manner of salutation this should be.
30 And the angel said unto her, Fear not, Mary: for thou hast found favour with God.
Luke 2:9-10
9 And, lo, the angel of the Lord came upon them, and the glory of the Lord shone round about them: and they were sore afraid.
10 And the angel said unto them, Fear not: for, behold, I bring you good tidings of great joy, which shall be to all people.
Notice especially that two of the above encounters are with the angel Gabriel.
While there are many accounts of angelic encounters in Scripture besides the ones above, what is absent from any of them is the angel squeezing, or attempting to further terrify the person that they are appearing to, in order to deliver good news - which in Muhammad's case, would have been some of the greatest news ever received by a person - that they are designated to be the final messenger of God to humanity.
Further, none of the prophets in the Bible ever attempted to kill themselves in a state of despair caused by confusion as to whether it was God, or actually a devil, who was communicating with them. And, many of them heard directly from God (or even saw God), rather than an angel. Additionally, there is no account of prophets receiving revelation in the manner Muhammad supposedly did, wherein he would sweat heavily (Sahih al-Bukhari 2661), snort (Sahih Muslim 1180a), breathe heavily (Sahih Muslim 1180c), have his face turn red (Sahih Muslim 1180c), and have his ears ring (Sahih al-Bukhari 2).
In addition to to Muhammad's initial suicidal despair after his first encounter with the entity in the cave, there is also a narration which states that after Waraqa died, and whenever revelation paused for extended periods of time, Muhammad would become suicidal again, and intended to throw himself off of a mountain repeatedly, with Gabriel appearing to him each time as he had done at first, and assuaging his grief:
Sahih al-Bukhari 6982
But after a few days Waraqa died, and the Divine Inspiration was also paused for a while, and the Prophet (ﷺ) became so sad as we have heard that he intended several times to throw himself from the tops of high mountains, and every time he went up the top of a mountain in order to throw himself down, Gabriel would appear before him and say, "O Muhammad! You are indeed Allah's Messenger (ﷺ) in truth", whereupon his heart would become quiet and he would calm down and would return home. And whenever the period of the coming of the inspiration used to become long, he would do as before, but when he used to reach the top of a mountain, Gabriel would appear before him and say to him what he had said before.
Notice above that the way in which Gabriel assuaged Muhammad's grief was by telling Muhammad that he was truly Allah's messenger. This seems to indicate that his anguish was caused by his own doubts as to whether or not he was actually a prophet of God, or merely possessed, as he had initially thought after his first encounter.
Muhammad's interactions with the entity which later identified itself as the angel Gabriel are nothing like the angelic encounters had by prophets of God in the Bible. Instead of the reassurance and immediate dispelling of fear seen in the angelic encounters in Scripture, the entity that Muhammad encountered assaulted him, amplified his fear, and then departed without identifying itself, or explaining completely what was happening, leading Muhammad to return to his wife in confusion and fear.
This account, in addition to the Satanic Verses Incident, and the incident of Muhammad's Bewitchment, undermine Muhammad's trustworthiness as a prophet, and demonstrate that he was not spiritually protected by God, and did not have contact with the God of Scripture.