This is once again a baseless allegation full of ignorance. The anti Ahmadis try to say that Hazrat Mirza Ghulam Ahmad(as) plagiarized the from Maqamat-al-Hariri which was known as the best poetry collection. Ahmad(as) claimed that Hujjatullah which means convincing proof from God, was in eloquent Arabic and would show the superiority of his Arabic. The non Ahmadis claim that Mirza Ghulam Ahmad(as) plagiarized but this is totally false as Ahmad(as) deliberately included some of the writings of Al Hariri with his own writings and challenged the critics to compare the two as his claim was that his Arabic was the most eloquent. The anti Ahmadis fail to understand that Mirza Ghulam Ahmad(as) was showing the superiority of his Arabic that one would not even be able to tell the difference between the two writings. Ahmad(as) makes it clear in the beginning of his book in Hujjatullah and the only reason He(as) did this was because the opponents mocked his Arabic. After this challenge none of them came forward. Never did the Promised Messiah(as) plagiarize and this allegation itself proves that the non Ahmadi Muslims have not read the books of Mirza Ghulam Ahmad(as).
Ahmad(as) states:
Thus the method which will free the people from his deception is that we present to him paragraphs from our writing and some other paragraphs from the writings of a great Arab writer while concealing the names of the authors, and then call upon him to tell us which paragraph out of this is ours and which is theirs, if you are truthful. Then if he recognizes my sayings and theirs and distinguishes between them as between a shell and its kernel, then we shall give him fifty rupees as a reward. ( Hujjatulla, pg. 4-5)