Sheikh Muhammad al-Turkî, professor at King Sa`ûd University
Regarding the book Kanz al-`Ummâl, if the questioner is asking if all hadîth mentioned in this book are authentic, then I would have to say: Definately not. The book is full of weak and false hadîth.
If you like to know where the author got the book’s narrations from then I can tell you that this book is, in fact, a rearrangement of al-Jâmi` al-Kabîr and its annexes that was originally compiled by Imam al-Sûyûtî. When Imam al-Sûyûtî wrote his book, his intention was to gather together all the hadîth that he knew, whether they were authentic or not. Al-Sûyûtî openly admitted that there were some false hadîth in it.
Therefore, when the author of Kanz al-`Ummâl rearranged al-Sûyûtî’s book, it was obvious that his book would also contain a number of weak and false hadîth, since the original work on which it was based contained the same.
Source: Islam Today