Ok, ansewring the OP: this is from “Vaterunser - Eine hilfe zum richtigen leben” by Anselm Grün. (The author is well known and celebrated.)
“The didaque is directed, also, to Christians of jewish origin, that were used to pray the prayer of 18 petitions 3 times a day. Instead of the jewish prayer, the Christians should now pray the prayer of Our Lord.” (6th paragraph of the introduction).
Sorry I can’t, I am pretty sure the author mentions it somewhere in the book but I can’t find exactly where. Also, the bibliography of the mentioned book is rather succinct and non of the cited authors seems to be jewish. But I am pretty sure this author knows what he is talking about and must have researched the matter. At least, I managed to answer the OP…Sorry if I can’t do better.
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