daf bit: Menachot 29
Rav Yehudah said in the name of Rav: when Moshe ascended Sinai to receive torah he found God affixing crowns to certain of the letters. (If you look in a sefer torah you'll see ornaments on the tops of some letters. That's what this is referring to.) Moshe asked: Master of the universe, is there anything wanting in torah that these crowns are necessary? God replied: after many generations a scholar named Akiva ben Yosef will expound upon each one of these crowns. Moshe replied: Master of the universe, please let me see this man! God said: turn around.
Moshe sat down behind eight rows of students and listened to Rabbi Akiva teach, but Moshe couldn't follow the arguments, which disturbed him. Then on one subject a disciple asked: how do we know it? Rabbi Akiva said: it is a law given to Moshe at Sinai. Comforted, Moshe returned to God and said: you have such a man as he and yet you give torah through me instead? God replied: be silent, for this is my decree. (Many tellings of this midrash end here.)
Moshe then said: Master of the universe, you have shown me his torah -- now please show me his reward. Turn around, said God, and Moshe saw them weighing out his flesh in the marketplace. Moshe protested: such torah, and such a reward?! God replied: be silent, for this is my decree. (29b)
(Akiva is one of the ten martyrs we read about on Yom Kippur. He famously died with the Sh'ma on his lips.)