וְאַתֵּנָה יְדַעְתֶּן כִּי בְּכָל כֹּחִי עָבַדְתִּי אֶת אֲבִיכֶן (בראשית לא, ו)
AND YOU WELL KNOW THAT I SERVED YOUR FATHER WITH ALL MY MIGHT. (BERAISHIS 31:6)
Just as an employer is cautioned against stealing the wage of the pauper [the employee] or withholding it from him, the employee is likewise cautioned not to steal from the labor due his employer by neglecting his work slightly here and there, spending the entire day in deceit… Similarly, he is obligated to put all his energy into his work, as the righteous Yaakov said: "I served your father with all my might." Therefore, he received the reward for this even in this world, as is written (Beraishis 30:43), "And the man became prodigiously wealthy." —Rambam, Laws of Sechirus 13:7
It is out of character for the Rambam, in a work strictly dedicated to documenting the laws of the Torah, to tell us of the reward that someone in history received for the observance of a particular law. Understandably then, the Rambam does not mention Yaakov's reward for putting full effort into his work as mere words of inspiration, but to explain and define this obligation.
The previous law taught by the Rambam, not to cheat your employer of the time for which he is paying you, is obviously a financial obligation between men and his fellow man. In contrast, the measure of working with all your might (besides for fully utilizing your time) is subjective, and would seem to be an ethical ideal, between you and G-d, more so than a duty between you and your fellow man. By adding the final words about Yaakov's reward, the Rambam dismisses this notion.
With the words "he received the reward for this even in this world", the Rambam is highlighting that not only was Yaakov rewarded for his righteous behavior, but that he "received the reward for this in this world": the earnings from his labor reflected his exceptional effort. Lavan had made a payment arrangement with Yaakov, and Yaakov felt duty-bound to do his work diligently in accordance with their agreement. Lavan had hoped to deceive Yaakov, but the wages that he was ultimately compelled to compensate him actually reflected Yaakov's hard work, making Yaakov an exceedingly wealthy man. By telling us that G-d ensured that Yaakov's wages from his employerwere commensurate to his diligence as an employee, the Rambam is teaching us that putting in one's full effort is not merely an ethical duty, but a financial obligation between employees and their employer.