Psaumes 19:12

for in observing them there is great reward I was careful in its observance because of Your great good that You have hidden away, and according to this usage, עקב is like (Gen. 26:5): “because (עקב) Abraham hearkened to My voice.” Another explanation of עקב רב: The end of the Torah scholars is that they will come to greatness. עקב is an expression of the end, and there is a similar expression in the language of the Mishnah (Sotah 9:15): “At the end (בעקבות) of the exile, prior to the coming of the Messiah.”

Moreover, by them is Thy servant warned: – meaning, I also am of them; although I am not among the wise, I am Thy servant and tremble at Thy commandments and am warned by them, and they are desired by and are sweet to me. Also I know that

In keeping them there is great gain: – Apart from the sweetness and the pleasure the wise man finds in them, the wise soul would pine and be eager for wisdom, even though it had not this recompense, for it is the nature of the wise soul to long, for wisdom, just as it is natural for the soul to long for sweet food. Such being the case, the wise busy themselves in Torah-study and in wisdom by nature, although they do not look for recompense in it. And David says: In keeping them there is great gain – more even than the desire. For to me and to the wise who desire them there is still further in the observance of them great reward, because Thou dost deal bountifully with those who observe the commandments both in this world and in the world to come; but the real reward is for the world to come. Therefore the reward is called עקב רב for as the heel (עקב) is the end of the body, so the reward is the end of work, and man's work is not finished until the day of his death.