Tur, Choshen Mishpat 337:13 (cf.Shulchan Aruch, Choshen Mishpat 337:20; Rambam, Hilchot Sechirut 13:7)

A worker is not entitled to work [elsewhere] at night and then hire himself out in the daytime, nor may he fast or mortify his flesh, nor feed his own food to his children, as these weaken him and render him unable to do the work of the employer properly (as the Rambam wrote). In the same way as an employer is not allowed to steal the wage of his employees nor delay their payment, so too the worker is not allowed to idle away his time, a little here and a little there, until his whole day is fraudulent. Rather, he must be punctilious with himself, since the wages were punctilious in freeing him from the collective grace after meals, and the fourth blessing of that grace [A person engaged in one mitzvah is free from the obligation to perform another]. Hence he is obligated to work with all his strength — as Jacob our ancestor said, “I have worked for your father with all of my strength” (Genesis 31:6) — so that he is paid what he deserves in this world, as it is said, “The man [Jacob] became extremely prosperous” (Genesis 30:43). [Ziegler translation]