The evidence for that is as follows:
It Is Not Permitted For Ijtihaad To Be Attributed To The Messenger Of Allah (saw)
1 – Ad-Daleel Al-‘Aqliy (Rational or intellectual evidence):
The concept of Ijtihaad is contradictory to the ‘Ismah (infallibility) of the Messenger (saw) because it is not permitted for error to occur in respect to the message (Ar-Risaalah) and conveyance (At-Tableegh). The Messenger of Allah (saw) is therefore infallible from an error in respect to the message and the conveyance and because we have been commanded to follow the Messenger (saw) in respect to that which he conveys to us from his Rabb. That is whilst Ijtihaad is open to error and it is not intellectually conceivable for Allah to command us to follow the error.
2 – Ad-Daleel Ash-Shar’iy:
Allah Ta’Aalaa has informed in the Qur’aan Al-Kareem that the Messenger (saw) only warns by the Wahi and only speaks or utters from the Wahi (divinely inspired revelation). Allah (swt) said:
Say: “I warn you only by the revelation” (Al-Anbiyaa’ 45).
And He Ta’Aalaa said:
Nor does he speak of (his own) desire. It is only a revelation that is inspired (An-Najm 4)
The realities proved this. That is because when an issue arose or an incident occurred, he delayed answering the question or passing judgment upon the reality, until the Wahi had been revealed upon him, some of which came to him quickly whilst some of it came after days. And so he did not undertake Ijtihaad in order to respond but rather he waited for the answer or the Hukm from the Wahi. Allah (swt) said:
I but follow what is revealed to me by inspiration (Al-An’aam 50).