The rational proof for the sinlessness of prophets is as follows:

The sending of the prophets and the showing of miracles in their lie. Also, it is an endorsement that they have the ability to communicate the message of God to their people. But a man who indulges in sins and such actions as would harm a project, cannot be said to be qualified to preach the usefulness of that project or to invite the people to participate in it. Therefore, when God showed miracles in the hands of the prophets it not only authenticated their claim of prophethood, but also confirmed that they could not commit error in receiving and delivering the divine revelation, and that they faithfully obeyed all the commands of God and desisted from all such things which were disliked by God. In other words, they were sinless in all its aspects.

‘Ismah which is generally translated as ‘‘sinlessness’’, concerned here, is of three kinds:

(1) Protection from error in receiving the revelation. 

(2) Protection from error in communicating and imparting that revelation to the people.

(3) Protection from sins. Sin here means any thing or action which goes against the obligatory command of God, and thus becomes a disgrace for the man. In other words, any word or action which is against the dictates of the servitude, homage and devoutness of man towards his Lord.

‘Ismah in religious terminology, means the presence, in the man concerned, of a quality which protects him from committing any thing unlawful, any error or sin.

The Qur’an proves all the above three types of protection as follows:

Protection from error in receiving the revelation and in its communication to the people is proven in the following verses: Knower of the Unseen; so He does not reveal His secrets to any except to him whom He chooses for an apostle; for surely He makes a guard to march before him, so that He may know that they have indeed delivered the messages of their Lord, and He encompasses what is with them and He takes account of everything (72:26-28). It is clear that God reserves His revelation for His messengers; so He reveals His secrets to them and supports them by guarding them from in front of them and behind them; and encompasses what is with them to protect the revelation from decay and alteration, whether that alteration comes from Satan or others; and it is done to make sure that they have indeed correctly delivered the messages of their Lord. 

Protection from sinning is proven in the following verses: These are they who, God has guided; therefore follow their guidance (6:90); Whomsoever God guides, he is the rightly guided one (18:17). According to these verses those who are guided by God can never be led astray by any misleading agency. In other words, they are free from all misguidance; and every sin is a misguidance, as is shown by the words of God: Did I not enjoin on you, O children of Adam! that you should not worship the Satan? Surely he is your open enemy, and that you should worship Me; this is the right way. And certainly he led astray a great multitude from among you. What! could you not then understand? (36:60-62). In these verses, God counts every sin as going astray, a misguidance as a result of the agency of Satan and also counting it as worshipping the Satan.

Looking at all such verses we can deduce the following:

(1) God guided the prophets through His Guidance.

(2) Anyone guided by God can never be misled, can never go astray.

(3) Every sin is misguidance; every sin is straying from the right path.

It follows that the prophets of God were sinless, free from every sin and protected from every mistake in receiving and delivering the revelation of God. This in short is the Qur’anic proof for the sinlessness of the prophets.

Poser: Since prophets are humans like us, therefore they can make mistakes or commit sins otherwise they would not be regarded as humans. 

Reply: The question is not whether they can or cannot make mistakes and commit sins but rather, did they make mistakes or commit sins? If they did, then it goes against the rational and Qur’anic proofs shown above.

Reference: Al-Mizan English Volume 3, pp. 173-178, first edition, WOFIS, 1986