Fact: Rashi’s father, Yitzchak, and the “Rebbi Yitzchak” whom Rashi cites are different people.
Background: The name “Rashi” is an acronym for Rabbi Shlomo Yitzchaki, i.e. Rabbi Shlomo, the son of Reb Yitzchak. But the Rebbi Yitzchak cited could not possibly be his father, because Rashi is citing a midrash that was written almost a thousand years earlier.
The famous commentary of Rashi on the first verse in the Chumash, in which he defends the right of the Jewish people to the Land of Israel, states: “Rebbi Yitzchak said: God need not have begun the Torah before the verse (Exodus 12:2), ‘This month shall be your [first] month,’ which is the first commandment that the Israelites were commanded….”
The suggestion that Rashi was quoting his father is simply mistaken. There is, however, a centuries-old manuscript that makes that claim. In the 17th century, Rabbi David ben Rav Shmuel Halevi (better known as the “Taz”) stated that as a youngster he saw an old manuscript that related the following: The question asked in the name of Rebbi Yitzchak is not found in any collection of midrashim; the only source is the commentary of Rashi. So where did it come from? According to the anonymous manuscript, Rashi wanted to honor his father, Yitzchak, and mention him at the beginning of his monumental work. However, Yitzchak was not a very learned scholar. Rashi therefore requested that his father ask a question about the first verse in the Torah. His father then asked the above-stated question and Rashi supplied the answer based on an existing midrash. 
The Divrei David rejects at least part of the story. He argues that Rashi’s father was a scholar, as evident from Avodah Zarah 75a (s.v. v’lo pligi) where Rashi adopts his father’s interpretation, and refers to him as “Abba mori menuchato kavod” “My father, my teacher, may he rest in dignity.” It is not clear whether, aside from the question of Rav Yitzchak’s scholarship, the Divrei David accepts the rest of the story in the manuscript.
However, two decades later, Rav Naftali Hertz ben Rav Shimon Ginzburg rejected the story for a more fundamental reason: both the question and the attribution to Rebbi Yitzchak can be found in the Yalkut Shimoni (beginning of Bo on Exodus 12:2, section 187 citing Midrash Tanchuma)! Ginzburg thus clearly shows that Rashi was merely citing the Tanchuma and the Amora Rav Yitzchak Nafcha. He strongly rejects the notion that Rashi was citing his father.
Rav Menachem Kasher in Torah Sheleimah cites the Tanchuma as well and then discusses the misconception regarding Rashi’s father. He observes that the Chida refutes the story in the manuscript by pointing out, similar to the Divrei David, that Rashi cites his erudite father in his Talmudic commentary; moreover, the Chida identifies the Zohar and the Yalkut as sources for the midrash containing Rebbi Yitzchak’s name. 
Without citing these earlier sources, a relatively recent work, the Leket Bahir,  simply notes that the Rebbi Yitzchak that Rashi cites was a Tanna and that Rashi was quoting a source found in a version of the Tanchuma printed in 5644 (1884). This sheds some light on how the original misconception arose: It seems the first printed copy of the Tanchuma, which appeared in 1522, relied upon a manuscript that quoted the midrash but did not attribute it to Rebbi Yitzchak. In Vilna in 1884, Shlomo Buber published another Tanchuma, using a different manuscript (known as the Oxford manuscript), which quoted Rebbi Yitzchak. Possibly Rashi, when writing his commentary, used a manuscript of the Tanchuma which quoted Rebbi Yitzchak as well; but others were not familiar with the midrash being credited to Rebbi Yitzchak since for nearly 300 years the printed version of the Tanchuma did not do so. 
Notwithstanding the important authorities who exposed this misconception, the error is still prevalent. In late 2001, I heard a well-known American pulpit rabbi state it at a Bat Mitzvah. Similarly, Richard Elliot Friedman explains that “A commentary is a very personal work.… Rashi conveys this with his very first words (Genesis 1:1)...and traditionally Rashi has been understood to be referring to his father.” Justin Lewis, in an on-line course,  wrote: “Rebbi Yitzchak is probably Rashi’s father. This shows the sweetness and humility of Rashi...he does begin by giving honor to his father.”
It may be possible to justify this misconception somewhat. It is possible that Rashi specifically chose a midrash that opens with a citation of Rebbi Yitzchak as a way of indirectly honoring his father. Rashi’s Biblical commentary is primarily composed of paraphrases of various Talmudic and midrashic sources. Yet very rarely does Rashi identify the author of a particular quote. Furthermore, Rashi often quotes Bereishit Rabbah in his commentary to Bereishit; he rarely cites Midrash Tanchuma. In this case, Bereishit Rabbah makes a similar statement, but in the name of Rabbi Yehoshua of Sichnin. It is possible that Rashi chose to cite this particular midrash, from this particular source, and decided to include the author’s name, as a way of honoring his father. 
1. Rashi lived 1040-1105. See Encyclopedia Judaica (EJ) 13:1558-1565 for details about his life and work. It is nearly universally agreed that Rashi’s father was named Yitzchak and that the name Rashi is an acronym for Rabbi Shlomo Yitzchaki. There were, however, those, who referred to him as Rav Shlomo Yarchi. Others use both names. The Seder Ha’dorot (by Rav Yechiel Halperin, b. 1660; p. 150 in the 5722 ed.) identifies Rashi as “Shlomo Yarchi bar Yitzchak from Troyes.” The Jewish Encyclopedia (1905; 10:324) says the appellation Yarchi was erroneously applied in the 16th century. It resulted from confusion between Solomon bar Isaac (Rashi) and Solomon de Lunel. Because of this, Lunel was mistakenly ascribed as Rashi’s birthplace (instead of Troyes), and it was then “translated” (Lunel=moon=yarchi) to become Rav Shlomo Yarchi. In the 18th century Giovanni Bernardo De Rossi (1742-1831; See EJ 5:1557-1558; Annales Hebreaeo-Typographici: Seculi XV and ab anno 1501 ad 1540, p. 51 and p. 52 in the 1969 Philo Press ed.) identified two works of Rashi as being by “Rav Salomonis Jarchi.” Although no longer popular, Yarchi seems to have been a well-recognized name for Rashi in the 19th century. It is discussed in the 1905 Jewish Encyclopedia whereas it is not even mentioned in the 1973 EJ. In the Occident (II:2 [May, 1844]), it states: “The age of Rashi, or more properly Rabbi Solomon ben Isaac, also known as Yarchi...” and in (II:5 [August, 1844]) it lists great teachers such as “Rab Ashi, Saadiah Gaon, Yarchi [Rashi], Aben Ezra....” In his bibliography in The Living Torah (Moznaim, 1991), Rabbi Aryeh Kaplan writes “Rashi—acronym of Rabbenu Shlomo (Ben Yitzchak) Yarchi. See Shem HaGedolim.”
2. In his supra-commentary to Rashi on the Bible, Divrei David to Genesis 1:1. Rabbi David Halevi (ca. 5346 -5427 ) is most famous for his commentary Turei Zuhav to the Shulchan Aruch. On his life and work see EJ 5:1354-1355. Divrei David was first published in 5449 (1689), then not re-published until 5642 . See the recent edition by H.D. Chavel, 5738 [1977-78], reissued in 5753 .
3. According to Chavel, maybe Genesis Rabbah 1:3.
4. Not s.v. pligi as written in Chavel edition, note 1, p. 2 of commentary. This is the only known explicit reference to his father in all of Rashi’s writings. Using this as a proof that his father was a scholar is a little troubling to me. It hardly seems possible that in his voluminous writings Rashi should refer to his illustrious and erudite father a grand total of one time. This argument is also raised by Avraham Grossman, Chachmei Tzarfat Harishonim (Jerusalem, 5757 ), 123-4, where he notes that there are those who question whether that “Rashi” on Avodah Zarah is not an addition by the Rashbam.
5. Naftali Seva Ratzon (5468 ; reprint, Ashdod, 5762 ), 1.
6. Not 149 as given by Spiegel in his article cited below.
7. Rashi preceded the Yalkut Shimoni, which was compiled by Rav Shimon Hadarshan in 13th century Frankfurt. But it is mostly a compilation of much earlier works, and in this case the Yalkut is citing the earlier Midrash Tanchuma.
8. See R.H. Albek, Mavo LaTalmudim (Tel Aviv, 5729 ), 252-253 for the identification of Rabbi Yitzchak. Some of these sources are found in the Bar-Ilan weekly English parashah sheet to Bereishit (1996) by Bo’az Spiegel, available at www.biu.ac.il/JH/Eparasha/bereshit/spg2.html.
9. In a sharp conclusion, he retorts that the anonymous author of the manuscript cited by the Divrei David should have begun his commentary from Exodus 12:2 rather than Genesis 1:1 and he would then not have made this error.
10. Entry 52 to Genesis 1:1, 13.
11. d. 1806; in Shem HaGedolim, entry on Rashi.
12. See Torah Sheleimah to Exodus 12:2 (#20) that in the Zohar to Exodus 39b a similar statement is brought in the name of Rabbi Yitzchak. However, Shir Hashirim Rabbah 1:4 (#29) has a similar statement attributed to Rav Yanai.
13. By Rav Yeshaya Halevi Weiss (Brooklyn, NY, 1990).
14. Apparently, the author of the “ancient manuscript” cited by Divrei David was not familiar with Buber’s version of the Tanchuma and hence accepted the fanciful tale regarding Rashi’s father. We can no longer make that mistake.
15. “Why I Wrote My Torah Commentary,” Moment, December 2001, 56-59, 72-76. He is a professor at University of California, San Diego.
16. Written for Kolel: The Adult Centre for Liberal Jewish Learning, found at www.kolel.org/tastytreats/texts2.1.html.
17. After suggesting this, I found it had already been proposed. The Seder Ha’dorot (see fn. 1) quotes the Divrei David and the objection of Naftali Seva Ratzon, and then offers this suggestion. It is also made by Rabbi Chavel in his edition of the Divrei David. Rabbi Menachem Mendel Schneerson, zt”l, also discusses what Rashi’s intention was in quoting Rabbi Yitzchak. He clearly states that it was not Rashi’s father and that Rashi’s intention may have been partially to honor his father. (Sichot in English, vol. 46, Bereishit 5751, 94-96).
Reprinted from JEWISH ACTION Magazine, Fall 5763/2002 issue