Ta’anis 8a : Reish Lakish prepared his Mishnayos forty times (paralleling the forty days that Moshe spent learning Torah) before going to study with Rabbi Yochanan.
Compare that to the time (see Kreisus 27a) Rabbi Yochanan wonders if Rabbi Elazar, another of his students, pays attention when he is taught about the acceptance of an animal for sacrifice if its value changed from the time it is designated until it has been ritually slaughtered.
A discussion in Eruvin (page 54b), deals with teaching habits of an earlier generation. Rabbi Eliezer states that a teacher is obligated to repeat his lesson to a student four times. This is derived from the fact that Aaron sat and listened to his brother Moshe teach each Torah lesson to him and the Elders four times.
Rabbi Akiva, a student of Rabbi Eliezer, says that a teacher should give a lesson as often as it takes until the student comprehends. Rabbi Akiva quotes a verse from Devarim chapter 31: V’lamdah es Bnei Yisrael (And teach the Children of Israel).
On this same page in Eruvin, the Gemara relates an incident where Rabbi Preida has a habit of giving the same lesson to a student four hundred times until he feels that the student knows it well.
[The numerical value of 400 is used a few times in Talmud to exaggerate a large number while making a point. For example, the Rabbis state that the size of the Land of Israel is 400 parsa’os, or kilometers.]
Rabbi Preida tells a student that he will need to cut short their lesson that day because he has to go do a mitzvah errand. The Rabbi proceeds to teach the student the same lesson “four hundred” times but the student does not grasp the information fully. Rabbi Preida asks the student why he does not get the point and he replies, “because when you tell me that you may need to cut short the lesson to go do a mitzvah, I lose my focus and keep thinking you will tell me that it is time to stop”. Rabbi Preida tells him, “’you should know that I will teach it to you another four hundred times until you totally understand it and then I will break for my errand!” And the student does go on to absorb the lesson properly.
A Bas Kol (Heavenly voice) calls out and says that Rabbi Preida needs to be rewarded for his patience in making sure his student understands the lesson. An angel tells Rabbi Preida that he is permitted to choose between receiving another four hundred years of life or that all the worthy people of his generation will receive the World To Come at the proper time. The Rabbi chooses the latter but Heaven decides to reward him with both a longer life and that the worthy people are not subject to Purgatory prior to their final reward. Commentators remark that there is an obvious mistake in the relating of this story- that Rabbi Freida was rewarded with 90 extra years and not 400.
On Chagigah page 9b a verse from Tanach is cited that raises a question as to a conflict of choice of words. Prophet Malachi, chapter 3, verse 18 states: V’shavtem Ur’isem bayn tzadik larasha, bayn ovayd Elokim la’asher lo avado‘([At the Great Day of Judgment] you will return and see the difference between a righteous person and an evil person, between one who serves G-d and one who doesn’t.) Now, is this not a little redundant? Is not a righteous person also one who serves G-d, and vice versa?
Hillel explains that of course a righteous person serves G-d. But, there is the extra effort that also counts. The one who studies his lesson one hundred one times shows greater devotion than the one who studies it one hundred times. If the number one hundred seems familiar, it is because the Rabbis say that we should try to utter one hundred blessings a day- that is expected of us. So to repeat a lesson one hundred times may be expected; to go the extra effort is one who serves the purpose Hashem intends for us in this world.
Does that mean if a person studies his lesson one hundred times and does not go that one extra effort he is considered not serving Hashem, albeit a righteous person? Hillel answers by giving an example of the market place at which to hire a donkey to go ten kilometers costs one zuz, but to go even just one extra kilometer, the donkey driver charges another zuz and not anything less. So, too, Hashem is teaching the lesson of the value of just even a little bit more of an effort in doing a mitzvah.
Rav Yehuda states in the name of Rav on Sanhedrin 91B: anyone who withholds teaching halacha to his student is as if he has robbed him of his rightful inheritance of his ancestors. Maharsha comments there in Sanhedrin the example of Rav Preida in Eruvin as to the exemplary action of the teacher to make sure his student understands halacha.