[A Jew] should make Torah his principal occupation and his work his casual one. He should minimize his business pursuits and occupy himself with Torah. And he should remove fleeting pleasures from his heart, and work each day enough to maintain himself. ... The rest of the day and night, he should occupy himself with Torah.
Rema, Shulchan Arukh, Yoreh De'a, 246:21, in "Separation from the Worldly (Perishut)"