As for the rational reason, it is because the Prophet PBUH used to wait for the revelation in many rules despite the urgent need to clarify the ruling of Allah . If ijthad was allowed for him he would not have delayed in giving the ruling but he would have performed Ijtihād. Because he used to postpone giving the ruling until the revelation was sent down. This indicates he did not make Ijtihād. It also indicates that it was not allowed for him to make Ijtihād. Had it been allowed he would not have put off giving the ruling despite the need to do so. Also, it is obligatory to follow the Prophet , if he exercised Ijtihād it would be possible for him to make a mistake. If he made a mistake we would be obliged to follow him so the matter would necessitate that we follow a mistake which is not valid because Allah did not order to follow a mistake. Furthermore, the Messenger PBUH is infallible (ma'sum) from making mistakes in the conveyance of the Message. It is absolutely impossible on his part to make a mistake in the conveyance of (Allah's Message). Since allowing the Messenger PBUH to make a mistake negates the (concept of) Messengership and prophethood. So the affirmation of Messengership and Prophethood determines that the Messenger is not allowed to make mistakes. Regarding the conveyance of the Message it necessitates that he is protected from making mistakes in the conveyance. So it is impossible on the part of the Messenger PBUH to err in what he conveys from Allah . Consequently, it is not allowed on his part to exercise Ijtihād. Everything conveyed by him from the rulings, in his saying, action and silence is revelation from Allah and nothing else.
It should not be claimed that Allah will not allow him to remain on the mistake. And that he will swiftly clarify it to him . This is because the mistake in Ijtihād when it occurs from the Messenger PBUH becomes fard on the Muslims to follow until the clarification comes. Then this clarification would have reestablished another ruling different to that of the first ruling. The Muslims would be ordered to follow this ruling and leave the former ruling which is a mistake. This is invalid, it is not possible on Allah's part the He order the people to follow a mistake and then order them to leave it and follow the correct one. Similarly, it is not allowed on the part of the Messenger PBUH that he conveys a ruling and then say to the people that this ruling is a mistake because it is from me, and the correct ruling is what has come to me from Allah and inform them that they should leave the first ruling because it is a mistake and inform them of the correct ruling.
It should not be said that this is a rational evidence for a Sharī’ah matter as that is not allowed, the reason is that the Sharī’ah matter requires a Sharī’ah evidence, since the Sharī’ah matter whose dalīl has to be only a Sharī’ah evidence is the Sharī’ah rule. As for beliefs, their evidence can be rational or a Sharī’ah evidence. The subject whether the Prophet is a mujtahid or not is from the beliefs and not from the Sharī’ah rules. So its evidence can be a rational or Sharī’ah evidence.The fact that it is not allowed for the Messenger PBUH to be a mujtahid is proven by the rational evidence and the Sharī’ah evidence. It is one of the beliefs.
It should not be claimed that the Messenger PBUH actually performed Ijtihād in various rules and that Allah did not recognise his Ijtihād, and that He corrected the messenger’s Ijtihād and revealed verses which clarified the correct opinion. That should not be claimed because the Messenger PBUH did not exercise any Ijtihād in conveying any rule of Allah . Rather what is proven by the Qur’ānic text and the sound Sunnah is that he used to convey to the people from revelation. He did not convey anything in terms of legislation, beliefs, rules and the like, except if it had come via the revelation and that he would wait until the revelation for a particular incident was not revealed.
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As for the verses that are cited by those who say that the Messenger PBUH actually performed Ijtihād, and in which they assume Ijtihād took place. There is no ayah in which Ijtihād took place. For example, His saying: