A major belief held by Muslims is that prophets are sinless. The website aboutislam.net summarizes the standard belief as follows:
Are the Prophets Sinless?
The Prophets of Allah were all infallible. That is, they did not, and could not, commit major sins. Allah prevented them from this.
Second, they only, rarely, committed minor sins, or human errors. This was followed by immediate repentance to Allah.
I checked this perspective on many Muslim sites, and this seems to be the general consensus. They could only sin a little bit, or make "mistakes" unwittingly, whereupon they would immediately repent.
Muhammad claimed to affirm the Bible repeatedly (Surah 3:3-4, 5:43-48, 5:68, etc.), as discussed at length in the last section of Why I'm Not a Muslim. Therefore the prophets under consideration for this sinlessness include all the prophets from the Bible, plus Muhammad.
Islam therefore portrays all of the Bible's prophets, and Muhammad, as comic-book-style heroes that could do no wrong. This is because the illiterate false prophet Muhammad who was re-writing history to make the prophets agree with him was only smart enough to write these one-dimensional characters that exist nowhere in reality.
The reality is, mankind is sinful. The reality is, every man born from Adam has a sin nature, and therefore, "every man at his best state is altogether vanity" (Psalm 39:5). Oh, unless you're like me - a prophet for my religion. Then you become a superman who never disobeys God (except accidentally, in minor cases).
Real human beings are complex. Good people do bad things, and bad people do good things. Islam dumbs down the message to that of a cartoon, so that the ignorant Muslims following the false religion know who the good guys and bad guys are supposed to be.
If you read the Quran, gone are nearly all the stories that show heroes of the Bible as blatant sinners. Instead, they are replaced by one-dimensional caricatures which are "good guys" fighting the "bad guys". The bad guys are hopelessly bad and bungling idiots, and the good guys are always righteous, in Muhammad's fake revisions of Bible stories that litter almost every page of the Quran. Neither act or speak like real human beings with personalities and free will (which is fitting for Islam).
This doctrine is evil because it exalts ordinary human beings to the level of God, as only God is good or sinless (Mark 10:18, etc.). It also goes against the Bible's teaching on what makes mankind prone to sin in the first place - they're in the lineage of Adam (1 Corinthians 15:22).
The only way that you could get a son of Adam to be sinless would be to take away their free will, as anyone in a fleshly body born from Adam will sin (1 John 1:8-10). But if Allah took away their free will completely, why would he allow them to make "mistakes", or commit "minor sins" at all?
Only Jesus Christ, born of a virgin, was sinless (2 Corinthians 5:21, Hebrews 4:15: 1 Peter 2:21-22: 1 John 3:5). No one else has ever been sinless, or good (Romans 3:23, Romans 3:10-12, Romans 5:12, Psalm 14:2-3, 53:2-3, Galatians 3:22, Ecclesiastes 7:20: 1 Kings 8:46), least of all the murderous, idolatrous, lustful, false prophet Muhammad.
More proof that Muslims hold to a sinless/infallible prophets doctrine is the fact that the Bible documents the sins of many of the prophets is used as a Muslim polemic against Christianity, as they see such accounts as slander.
In the actual Word of God, we see that David committed adultery and murder. Moses murdered an Egyptian for a non-capital offense (beating a slave). Paul put Christians to death before he was saved. The last we hear of Solomon, he was living as an idolater with 1,000 wives. Sampson and Saul killed themselves after falling out with God.
Lot moved to a wicked city for money, and later committed incest while drunk. Abraham lied everywhere he went and led people to think his wife was his sister, and his son Isaac followed in his footsteps. Jacob lied to his father, and swindled his brother Esau. Peter denied the Lord. Thomas doubted Jesus rose from the dead. Jeremiah got mad at God and tried to resign as a prophet. Noah got extremely drunk, and an entire nation got cursed as a direct result.
You affirm the Bible, Muhammad? Which Bible? The one you invented out of thin air, where prophets are not human sinners in sinful bodies of decaying flesh? The one that no one has ever seen or can check, and only exists in the deluded minds of Muslims?
What Muslims are alleging, if we carry their accusation to its conclusion, is that for some reason, the Jews and the Christians corrupted the Old Testament to add, for instance, the account of David's adultery and murder. Why? To slander David, who we both reckon as one of the greatest prophets to ever live, despite his failings? To justify adultery and murder? Then why did we also "corrupt" the text to include God's condemnation of the adultery and murder? Accounts like these actually prove that our text is genuine, and prove Islam to be a fictional, sanitized corruption of our religion.
Think, Muslims - which is more likely to be made up? Do you think the Jews, when going to write the history of their kings, thought, "Well, we have to be honest and document what he did wrong". Or, do you think they thought "Our king was utterly perfect, yet let's make up horrible things about him", and then Muhammad, who never met any of them, sets us all straight over 1,500 years later? Islam's revised stories of the prophets fail the principle of embarrassment, and the Bible's honesty shows that it's the Word of God.
Discounting the fact that according to the Bible, Muhammad was an exceedingly evil sinner, even according to Islam and himself, he was a sinner.
Three times, explicitly, in the very text of the Quran, Muhammad is commanded by God to seek forgiveness for his sins, and is otherwise said to be a past and future sinner:
Surah 40:55 (Haleem)
55 So be patient, Prophet, for what God has promised is sure to come. Ask forgiveness for your sins; praise your Lord morning and evening.
Surah 47:19 (Haleem)
19 So [Prophet], bear in mind that there is no god but God, and ask forgiveness for your sins and for the sins of believing men and women. God knows whenever any of you move, and whenever any of you stay still.
Surah 48:1-2 (Haleem)
1 Truly We have opened up a path to clear triumph for you [Prophet],
2 So that God may forgive you your past and future sins, complete His grace upon you, guide you to a straight path,
Why do I have to include the translator's name above? Because "sins" has been softened to "shortcomings" or "mistakes" in some translations by liars hiding behind the Arabic, who habitually lie in order to make their false religion look better. They are trying to cover the fact that Muhammad was a filthy, dirty, fallible, wicked, sinner.
Muhammad also states the following in Sahih al-Bukhari:
Sahih al-Bukhari 6307
Narrated Abu Huraira:
I heard Allah's Messenger (ﷺ) saying." By Allah! I ask for forgiveness from Allah and turn to Him in repentance more than seventy times a day."
He was repenting seventy times per day because he was a sinner. Other Hadith that mention Muhammad's sins, or him praying for forgiveness for his sins, include Sahih al-Bukhari 20, 1120, 2397, 6377, 7385, 7442, 7499, Sahih Muslim 483, 771a-b, Sunan an-Nasai 897, 5466, Sunan abu-Dawud 761, 878, and Jami at-Tirmidhi 3421-23.
According to himself, then, he was a sinner. According to the Bible, he was a big sinner. According to Allah in the Quran, he was a sinner. However, Muslims deny all common sense and reason by pretending he wasn't (at least not a "big" sinner). Seventy times per day?
This is in stark contrast to Jesus Christ, who never sinned, and challenged anyone to convict Him of a single sin (John 8:46) while He was living amongst other humans here on this earth - who could watch Him 24 hours per day. And yes, that means not even "minor" sins, "on accident", which are just alibis for the fact that Muslims are attempting to declare sinners to be sinless.
Related to the doctrine of the infallibility of the prophets is the teaching that their bodies do not decay after death:
Sunan abu-Dawud 1047
Narrated Aws ibn Aws:
The Prophet (ﷺ) said: Among the most excellent of your days is Friday; on it Adam was created, on it he died, on it the last trumpet will be blown, and on it the shout will be made, so invoke more blessings on me that day, for your blessings will be submitted to me.
The people asked: Messenger of Allah, how can it be that our blessings will be submitted to you while your body is decayed? He replied: Allah, the Exalted, has prohibited the earth from consuming the bodies of Prophets.
This is also narrated in Sunan abu-Dawud 1531.
The bodies of prophets don't decay? Joseph's did (Genesis 50:24-26, Exodus 13:19-20). Elisha's did (2 Kings 13:20-21). This lying false prophet twisted Psalm 16:10, which was about Jesus, and tried to apply it to non-virgin-born sinners, including himself.
This brings us to a remarkably easy way to prove Islam. Muhammad's tomb is in Al-Masjid al-Nabawi in Saudi Arabia. Let's dig his body up, and we can see his body perfectly preserved, proving once and for all that Islam is true.
Of course, the Muslims won't do this, because the lying, Satanic false prophet Muhammad rotted like a sliced apple when he died, just like every other sinner to ever live, and the Saudis know that, so they don't pull a Vladimir Lenin, and put Muhammad on display. This proves that the Communist government of Russia is better at preserving corpses than the false god Allah.
Islam exalts man, diminishes Jesus Christ, and refutes itself, by clinging to these false doctrines.
They are doctrines taught by people who are holding out hope that maybe the noblest of mankind can be perfect in God's eyes without Jesus Christ's atonement. It's a way to deny that Jesus is the only way for any son of Adam to be made righteous before God (John 14:6), before or after the sinner Muhammad showed up and spouted off his mouth.