What I found is that written evidence for knocking on wood (and for the history of gesture as a whole) is remarkably sparse. In the Penguin Guide to Superstitions, folklorist Steve Roud offers a skeptical take on the provenance of knocking on wood due to the near total absence of written records about it:
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Москвичей предупредили о резком похолодании09:45
（三）违反本法第四十五条至第四十六条的规定，未依法履行网络安全产品、服务备案等义务的；