Musar על מגילה 55:21

And the Torah leads one to good deeds. For when a man occupies himself with Torah, and learns what is the punishment for sins and the reward for good deeds, then he sets his heart to do good. And the effort required in studying the Torah causes him to forget and restrains him from sin, while idleness leads to sin and brings him down to the nether world. Therefore, a man should be alert to occupy himself always with the Torah — day and night — even when he walks along the road or when he lies in his bed. And if he is not able to study by heart, let him think in his heart about what he has learned and let his mind be on his study, so as not to remove the thought of it from his heart, and then he will have fulfilled "But thou shalt meditate therein day and night" (Josh. 1:8); for the Scripture does not say, "You shall speak about it day and night," but "You shall meditate about it," and meditation is in the heart. And concern ing this it is said, "And shalt talk of them when thou sittest in thy house, and when thou walkest by the way, and when thou liest down, and when thou risest up" (Deut. 6:7, 11:19). And our Sages took pride in the fact that they did not walk four cubits without discussing the Torah. Either they would learn and study or they would think in their hearts about what they had heard concerning the Torah, or about their studies.