In my opinion, the first father to state what we call the "regulative principle of worship" was Clement of Rome, who wrote,
"Let every one of you, brethren, give thanks to God in his own order, living in all good conscience, with becoming gravity, and *not going beyond the rule of the ministry prescribed to him.* Not in every place, brethren, are the daily sacrifices offered, or the peace-offerings, or the sin-offerings and the trespass-offerings, but in Jerusalem only. And even there they are not offered in any place, but only at the altar before the temple, that which is offered being first carefully examined by the high priest and the ministers already mentioned. Those, therefore, who do anything beyond that which is agreeable to His will, are punished with death. You see, brethren, that the greater the knowledge that has been vouchsafed to us, the greater also is the danger to which we are exposed."
Again, I would tend to read that a bit differently than as a statement of the RPW, normally defined. Clement's subject, as noted in the preceding section of the letter, did concern not performing "sacrifices and service" "thoughtlessly or irregularly," but "at the appointed times and hours" and "where and by whom [God] desires these things to be done." Clement's main thrust is that the laity respect the person and role of their presbyters. Though not completely separate, that seems somewhat distinct from many of the core elements of worship (words, actions) addressed in the RPW.
