Many doctors (mujtahid) of early community and those who followed them have held or practised what was innovation, not knowing that it was so, either because of weak ahadith they thought sound, or because of understanding from a Qur'anic verse what does not follow, or for some personal view held in ignorance of definitive texts on the matter. If (such) a man fears Allah as much as possible he may still hope for mercy.
The point here is that the Messenger has clearly shown all of religion in the scripture and in his sunnah. Ijma' - the joining together of the community - is right, and it shall not join together on an error.
The verse cited as a proof-text for 'joining together' is:
"But him who breaks with the Messenger after guidance has become clear to him, and follows other than the way of the believers, him We shall consign to what he had turned to, and roast in Jahannam - an evil home - coming."
We may sum up the possible exegeses of this thus: The blame attaches to those who break with the Messenger only, or those who follow other than the way of Believers only, or else blame does not attach to one of these but to both of them, because they are closely connected with each other. The first two positions are false, because if only one of the two categories is affected, there would be no point in mentioning the other. That the blame should not attach to one of these two categories is absolutely untenable.
Breaking with the Messenger clearly entails the divine threat, apart from anything else. Attaching blame to both categories but treating them as separate is not indicated by the verse; it concerns both together.
There remains the last position, which is that all those described thus incur the threat, because they are closely connected, just as the like is said of those who rebel against Allah and the Messenger, and as it is said (in the hadith) that one who opposes the Qur'an and Islam or leaves the Qur'an and Islam is of the people of Hell, and Allah's saying:
"He who disbelieves in Allah and His Angels, His Books, and His Messengers has strayed into extreme error, so that disbelief is each of these fundamentals involves disbelief in the others, and whoever disbelieves in Allah has disbelieved in all of them."
It is similar with breaking with the Messenger and following other than the way of the Believers; whoever breaks with him has followed other than the way of the Believers - that is obvious. And whoever follows other than the way of the Believers has broken with him, and exposed himself to divine threat. Any one departing from their 'joining together' has followed another way absolutely, and accordingly exposed himself to the Divine threat. If anyone says they are only blamed because at the same time they broke with the Messenger, we say, 'yes' - because the two acts are intimately connected, for the reason that whatever the Muslims join together on must be based on texts from the Messenger of Allah, so that whoever opposes them has opposed also the Messenger, and whoever opposes him, opposes Allah. It necessarily follows that everything on which they join together should have been demonstrated by the Messenger, and that is our point.
Thus there is absolutely no question on which they are joined together that has not been clearly demonstrated by the Messenger, but this has escaped some people, and he know about 'joining together' and try to use it as proof, as one would use a text, if they do not know any proof texts. However 'joining together' is a second proof, to be added to the text, like an example given in the Qur'an. Whatever joining together indicates has also been indicated by scripture and the Prophet's sunnah. All that the Qur'an indicates has come by the way of the Messenger, since both the Qur'an and sunnah came from him, so there cannot be any question on which 'joining together' has occurred, unless it has been based on a text.
Many ulama have not known the texts, and still have concurred with the collectivity, just as they have used logic or a matter where 'joining together' had occurred without their knowing it, and still agreed with the consensus.
Whatever modern says that 'joining together' is the basis of the greater part of the Law has given himself away, for it is lack of knowledge of the scripture and the sunnah that drives him to say it. Similarly when they say that most events require use of logical analogy, because there is no indication in the texts - that is only the statement of one who has no knowledge of the Book and the sunnah with their clear rules for making judgements.
The sunnah cannot abrogate the Book. If there be anything abrogated in the Qur'an, the abrogation is written there, since nothing can take precedence over it. If one does not find something in it, he may look for it in the sunnah, and nothing can abrogate a hadith except another hadith: the sunnah is not abrogated by 'joining together' or anything else. For joining together cannot contradict scripture or the sunnah.
From Ladders of Attainment