In Sahih al-Bukhari, there are many narrations which testify to the fact that Muhammad, long after becoming a prophet, was bewitched, having fallen victim to black magic:
Sahih al-Bukhari 3175
Narrated Aisha:
Once the Prophet (ﷺ) was bewitched so that he began to imagine that he had done a thing which in fact he had not done.
The nature of the bewitching, puzzlingly, related to him thinking that he had intercourse with his wives, when he had not:
Sahih al-Bukhari 5765
Narrated Aisha:
Magic was worked on Allah's Messenger (ﷺ) so that he used to think that he had sexual relations with his wives while he actually had not (Sufyan said: That is the hardest kind of magic as it has such an effect).
Then one day he said, "O `Aisha, do you know that Allah has instructed me concerning the matter I asked Him about? Two men came to me and one of them sat near my head and the other sat near my feet. The one near my head asked the other. 'What is wrong with this man?' The latter replied 'he is under the effect of magic'. The first one asked, 'Who has worked magic on him?' The other replied 'Labid bin Al-A'sam, a man from Bani Zuraiq who was an ally of the Jews and was a hypocrite.' The first one asked, 'What material did he use?' The other replied, 'A comb and the hair stuck to it.' The first one asked, 'Where (is that)?' The other replied. 'In a skin of pollen of a male date palm tree kept under a stone in the well of Dharwan'."
So the Prophet (ﷺ) went to that well and took out those things and said "That was the well which was shown to me (in a dream). Its water looked like the infusion of Henna leaves and its date-palm trees looked like the heads of devils." The Prophet (ﷺ) added, "Then that thing was taken out". I said (to the Prophet (ﷺ)) "Why do you not treat yourself with Nashra?" He said, "Allah has cured me; I dislike to let evil spread among my people."
This narration is also given in Sahih al-Bukhari 3268, 5763, 5766, 6063, and 6391.
The above account is very telling. It shows that Muhammad himself was the one claiming that a magic spell had been cast on him, that the spell had worked, and that he was able to get rid of the spell by removing some of his hair, which had been used in the magic, from a well.
And, as also pointed out in the article Muhammad Tortured and Killed a Man for Money, this is an instance of Muhammad not even taking his own wrong advice on how to deal with magic or poison:
Sahih al-Bukhari 5445
Narrated Sa`d:
Allah's Messenger (ﷺ) said, "He who eats seven 'Ajwa dates every morning, will not be affected by poison or magic on the day he eats them."
This is also narrated in Sahih al-Bukhari 5768, 5769, 5779, Sahih Muslim 2047a-b, and Sunan abu-Dawud 3876.
During the year that he was bewitched, he didn't remember his own revelation about dates being the cure to magic and poison? Why not? Could it be because he was an ignorant, superstitious man without divine revelation?
In the Old Testament, the Lord gave us the principle that so long as you are innocent, any curse that someone attempts to curse you with will be inert:
Proverbs 26:2
2 As the bird by wandering, as the swallow by flying, so the curse causeless shall not come.
Above, we see that like birds flying around without landing, no one's curse on an innocent person is going to come to pass.
As a demonstration of not only this principle, but also the divine protection that God bestows on believers, we can look at the story of Balaam in Numbers 22-24, who was a prophet of God, and was entreated by the enemies of Israel to curse them:
Numbers 22:5-6, 9-12
5 He sent messengers therefore unto Balaam the son of Beor to Pethor, which is by the river of the land of the children of his people, to call him, saying, Behold, there is a people come out from Egypt: behold, they cover the face of the earth, and they abide over against me:
6 Come now therefore, I pray thee, curse me this people; for they are too mighty for me: peradventure I shall prevail, that we may smite them, and that I may drive them out of the land: for I wot that he whom thou blessest is blessed, and he whom thou cursest is cursed.
9 And God came unto Balaam, and said, What men are these with thee?
10 And Balaam said unto God, Balak the son of Zippor, king of Moab, hath sent unto me, saying,
11 Behold, there is a people come out of Egypt, which covereth the face of the earth: come now, curse me them; peradventure I shall be able to overcome them, and drive them out.
12 And God said unto Balaam, Thou shalt not go with them; thou shalt not curse the people: for they are blessed.
Three times, Balaam is coaxed to curse Israel, and three times, he blesses them instead (Numbers 24:10), once saying:
Numbers 23:20-23
20 Behold, I have received commandment to bless: and he hath blessed; and I cannot reverse it.
21 He hath not beheld iniquity in Jacob, neither hath he seen perverseness in Israel: the Lord his God is with him, and the shout of a king is among them.
22 God brought them out of Egypt; he hath as it were the strength of an unicorn.
23 Surely there is no enchantment against Jacob, neither is there any divination against Israel: according to this time it shall be said of Jacob and of Israel, What hath God wrought!
Why didn't enchantment, or divination, work on Israel? Because God was protecting them. Why didn't God protect Muhammad, who was, according to Islam, far more important than the average Israelite, from enchantments? Because he was a false prophet.
Lastly, we can look at the promise of divine protection given to Abraham, and passed on through Christ to all believers:
Genesis 12:1-3
1 Now the Lord had said unto Abram, Get thee out of thy country, and from thy kindred, and from thy father's house, unto a land that I will shew thee:
2 And I will make of thee a great nation, and I will bless thee, and make thy name great; and thou shalt be a blessing:
3 And I will bless them that bless thee, and curse him that curseth thee: and in thee shall all families of the earth be blessed.
According to the above, anyone who cursed Abraham would only succeed in cursing themselves. In the New Testament, we learn that this promise was made to Christ, through Abraham, and carried on through Him to all believers:
Galatians 3:16, 29
16 Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
29 And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.
Why didn't Muhammad inherit the above promise? Because he was not a successor to Christ, but instead a false prophet, who invented brand new revelations while trying to ride on the coattails of the great prophets in the Bible.
Whereas Jesus Christ constantly demonstrated His control over the spirit world in His ministry, the false prophet Muhammad, according to himself, in Islam's own sources, was affected by black magic, performed on him by a Jew that had gotten ahold of some of his hair.
Muslims have to reckon with why God would allow Muhammad to be affected by petty black magic, despite all of what God already said in the Bible about curses, especially in light of the fact that Muhammad claimed to be the seal of the prophets.