That's from the Introductory Epistles to/among the Clementine Homilies: the pseudo-Clementina
There's two there, both titled 'Epistle of Peter to James'
That one, the first one, starts
Knowing, my brother, your eager desire after that which is for the advantage of us all,
I beg and beseech you not to communicate to any one of the Gentiles the books of my preachings which I sent to you, nor to any one of our own tribe before trial; but if any one has been proved and found worthy, then to commit them to him, after the manner in which Moses delivered his books to the Seventy* who succeeded to his chair. Wherefore also the fruit of that caution appears even till now. For his countrymen keep the same rule of monarchy and polity everywhere, being unable in any way to think otherwise, or to be led out of the way of the much-indicating Scriptures. For, according to the rule delivered to them,
they endeavour to correct the discordances of the Scriptures, if any one, haply not knowing the traditions, is confounded at the various utterances of the prophets. Wherefore they charge no one to teach, unless he has first learned how the Scriptures must be used. And thus they have amongst them one God, one law, one hope.
https://www.newadvent.org/fathers/080800.htm
* Numbers 11 (vv.16–17, 24b–25) is concerned with the handing on of power: seventy elders are appointed to transmit Moses’s authority to future generations (it raises the prospect of being behind the legend of 70 scribes of the Septuagint/LXX)
The next chapter:
In order, therefore, that the like may also happen to those among us as to these Seventy, give the books of my preachings to our brethren, with the like mystery of initiation, that they may indoctrinate those who wish to take part in teaching; for, if it be not so done, our word of truth will be rent into many opinions. And this I know, not as being a prophet, but as already seeing the beginning of this very evil. For some from among the Gentiles have rejected my legal preaching, attaching themselves to certain lawless and trifling preaching of the man who is my enemy. And these things some have attempted while I am still alive, to transform my words by certain various interpretations, in order to the dissolution of the law; as though I also myself were of such a mind, but did not freely proclaim it, which God forbid! ...
