Мидраш к Ийова 5:12

Ulla said: "Trouble [about sustenance] affects the memory, even for the words of the Torah, and makes one forget what he studies, as it is said (Job 6, 12) Who frustrateth the plans of the crafty, so that their hands cdnnot execute their well-devised counsel." Rabba said: "If, however, he occupies himself with the Torah for the sake of Heaven, it will not affect him, as it is said (Prov. 19, 21) There are many thoughts in a man's heart; but the counsel of the Lord alone will stand firm; i.e., study which is for the sake of Heaven, no matter in what circumstances one is, remains forever."