"And let him write a second copy of this Torah" [Devarim 17:18].
The name of the book of halacha written by the Rambam – "Mishneh Torah" – comes from the above verse. The book is a compilation of all the halachot that appear in the Oral Torah. It is commonly accepted that the Rambam did not write any law unless he had found a source for it in the words of the sages. For this reason, many Torah scholars have spent their time throughout the generations and many books have been written in an effort to explain the methods of the Rambam and to find his sources within the Talmud – both Babylonian and Yerushalmi – and in the Midrash. But there are still some halachot which remain hazy and for which no sources have been found.
One example of the above is the following: "The early wise men said: Anybody who becomes angry can be compared to one who worships an idol" [Hilchot De'ot 2:3]. What appears in the Talmud is, "One who shatters vessels in his anger can be compared to one who worships an idol" [Shabbat 105b]. Where did the Rambam see that this statement is true about "whoever becomes angry"? And just who are the "early wise men"?
Rabbi Reuven Margaliyot, the first director of the Rambam Library, was nicknamed "the minister of the Zohar" in view of his great expertise about this book. The edition of the Zohar with his notes "Nitzutzei Zohar" is a treasure trove of comparisons and references from the Zohar to all parts of the Torah. Rabbi Margaliyot wrote a series of articles on the subject of the link between the Rambam and the Zohar which were published in the journal "Sinai" in the years 5713-5714 (1953-54). With respect to the above halacha about anger, Rabbi Margaliyot shows sources in the Zohar, the Tikunei Zohar, and in Zohar Chadash (Midrash Ne'elam), in the Torah portions of Bereishit, Korach, and Pinchas. In all, he gives six sources where the phrase quoted by the Rambam appears.
Does this prove that the Rambam saw a copy of the Zohar?
There are various opinions about this matter, and this article is too short to give them all in detail. From among the scholars in our generation, we will quote the opinions of two rabbis who are known for their encyclopedic knowledge of sources, Rabbi Chaim Kanievsky from Bnei Berak and Rabbi Ovadia Yosef.
Rabbi Chaim Kaneivsky wrote the book "Kiryat Melech," which gives sources for all the rulings of the Rambam in his monumental work. He often gives the Zohar as a source for the Rambam. For example, in Chapter 4 of Hilchot Yibum V'Chalitza the Rambam writes that after the end of Chalitza (when a brother-in-law refuses to marry his brother's widow), the woman throws the shoe that was used in the ritual onto the ground. Rabbi Kanievsky quotes from the GRA that the source for this custom is in the Zohar for the portion of Chukat. It should be noted that at the end of his book, Rabbi Kanievsky writes as follows: "I have made reference to the Zohar, even though the Rambam did not see it... It is possible that he quoted from the words of the Geonim, who did see the Zohar, and it is also possible that the Rambam saw the Midrash Ne'elam."
Rabbi Ovadia Yosef discussed this matter twice. The first is a short note in a halachic responsa about the writing of the holy name on the outside of a mezuzah (Yavia Omer volume 8, Yoreh Dei'ah 28). Rabbi Yosef completely rejects the idea that the Rambam saw the Zohar, based on the words of the Chida: "There are those who say that the Rambam wrote various halachot based on the words of the Zohar. This does not seem to be true, since the Zohar was not even revealed to the Rashba and the ROSH, and certainly not in the days of the Rambam." However, Rabbi Yosef returns to this subject in his (relatively) new book "Tabaat Hamelech," on the subject of the Mishneh Torah of the Rambam (Jerusalem, 5765). In this case he expanded the discussion based on broad knowledge of sources, as he tends to do, with respect to all the halachot of the Rambam that are attributed by recent rabbis as having been based exclusively on the Zohar. After checking through every one of the references, he writes: "It is possible that some parts of the Zohar existed in ancient times, in view of the fact that the 'Aruch' quotes items in the name of the Midrash of Rabban Shimon Ben Yochai which also appear in the Zohar."