I heard in a Shiur the rabbi mention that there’s an opinion in the Gemara that Mashiach came and went already. Does anyone know where this is from, what the rabbi’s proof is, and what commentaries have to say on it?
"ר' הילל אומר אין להם משיח לישראל שכבר אכלוהו בימי חזקיה"
"Rabbi Hillel says: There is no Messiah coming for the Jewish people, as they already ate from him during the days of Hezekiah."
As can be seen, Rabbi Hillel did not expound on this idea. Rav Yosef was quick to refute this idea later on in the gemara in 98b.
Commentators have presented different explanations for this strange statement. Rashi, for example, suggested that Rabbi Hillel believed that there won't be a human Mashiach but rather Hashem Himself will rule over His people.
Some modern discussions of the quote:
Rabbi Menachem Kasher discussed this quote at length in his introduction to Kol Ha'Tor, pp. 155-180 and brings many understandings of this idea, as well as discussing Rashi's interpretation and bringing similar sources.
@Deuteronomy already brought in the comments the pretty well-known piece by Rabbi Gil Student on the topic. Rabbi Student brings five opinions of four Rishonim and one early Acharon on how to understand Rabbi Hillel's view in a manner that is not heretical. The gist of these opinions is that Rabbi Hillel did believe in some form of the days of Mashiach, but a form that is different from what is now the common belief.
Rabbi Gavin Michal also discussed this on his blog. He brings a mix of modern scholarly opinions and, based on Eric Lawee's essay, a number of Sephardic interpretations of the quote. Some of these opinions do in fact hold that Rabbi Hillel's statement was heretical, but others view it in an optimistic light, that Mashiach will come shortly.
And Emmanuel Levinas discussed this quote in his book Difficile liberté: essais sur le judaïsme (pp. 81-84 in Difficult Freedom (trans. S. Hand)).
As a side-note, I myself am in the middle of formulating a theory on this mysterious quote to connect it with some particular historic events. I might update this answer if I have enough of a base for my idea.
Artscroll Sanhedrin 99a1 note 13 explains
R' Hillel does not dispute that the Jews will be redeemed from exile. Rather, he maintains that the redemption will not be wrought by a human messiah, but by Hashem himself (Rashi)
According to Yad Ramah, R' Hillel maintains that the redemption promised in the Torah occurred when Hashem saved the Jews from Sancheruv in the time of Chizkiah.
Ran says that this means that the future redemption will be done directly by Hashem including techiyas hameisim (resurrection of the dead).
He maintains however, that as soon as Hashem has redeemed us, He will resurrect the dead and they will benefit from the radiance of His presence. There will be no physical utopia intervening between the redemption and the resurrection, for that period already came to pass during the reign of Chizkiah. When R' Hillel said "they already enjoyed [the Messianic Era] in the days of "Chizkiah, he was only referring to the physical aspect of the Messianic Era (see 99b3 note 31) - the spiritual aspect is yet to be fulfilled
This is discussed in detail in Sefer Yeshuot Meshicho by Abarbanel in the second Iyun (deep review) citing Yerushalmi Brachot, chapter 2, Mishnah 4 toward the end.
The Talmud Yerushlmi says:
Rebbi Joshua ben Levi said, his name is Tzemach. Rebbi Yudan, the son of Rebbi Aivu, said his name is Menachem. Chanina the son of Rebbi Abbahu said: they have no disagreement since the numerical value of one is the numerical value of the other: Tzemach is equal to Menachem (צמח = מנחם). Since Rebbi Yudan the son of Rebbi Aivu said, it happened to a Jew who was plowing in the valley of Arbel that his ox was bellowing. An Arab passed by and heard the bellowing of the ox. He said to him: Jew, Jew, unharness your ox, unharness your plow because the Temple was destroyed. The ox bellowed a second time. He said: Jew, Jew, harness your ox, fix your plow because King Messiah has been born. He said to him: What is his name? Menachem. He said to him: What is his father’s name? Chizqiah. He said to him: Where is he? He said to him: At the king’s palace in Bethlehem in Judea. He went and sold his ox and plow and made himself a vendor of baby linens. He went to towns and left towns until he came to that town. All the women bought from him but the mother of Menachem did not buy. He heard the women say: Menachem’s mother, Menachem’s mother, come and buy for your son! She said: I would rather strangle him like a hater of Israel because he was born on the day the Temple was destroyed. He said to her: I like him, because for him it was destroyed and for him it will be rebuilt. She said to him, I have no money. He said to her, that does not bother me, come and buy for him! If you have nothing with you today, I shall return another day and I will take it then. After some time he went up to that town and said to her: How is your baby doing? She said to him: After you had seen me there came storms and raised him and tore him from my hands. Rebbi Abun said: We do not need to learn from that Arab, is it not a full verse (Is. 10:35): “The Lebanon will fall through a noble one”? And the next verse is (11:1) “A sprout will come from the stem of Jesse.”
Among other things, Abarbanel explains that the Arab mentioned here was Eliyahu HaNavi and that this entire episode is prophesy.