Deuteronomy 15:18

18.It shall not seem hard unto thee. I have lately observed how difficult and inconvenient to the Jews was the observance of this law; wherefore it is not without reason that God reproves their mean and niggardly pride, if they enfranchised their slaves grudgingly. And, indeed, He first urges them to obey on the score of justice, and then from the hope of remuneration. For He reminds them that for six years the slave had earned double the wages of a hireling, either because his life was more laborious, inasmuch as heavier tasks are required from slaves than from free-men, who are paid for their work; or because he had completed twice as long a period as hirelings were wont to be engaged for. For the Jewish (commentators) (149) infer from this passage, that three years was the term prescribed for hired servants; and thus they suppose the six years were counted. But since this is a mere conjecture, I know not whether my opinion is not more suitable, that for six years their labors had been twice as profitable as would have been those of a free-man who is not under the compulsion of a slave.