Midras Amsal 6:31: Mekhilta DeRabbi Yishmael

But if one steals away from his friend, (who asks to be paid for teaching him), and goes (and hides behind a fence) to learn Torah (i.e., to overhear the lesson that he is teaching), though he is called a "thief," he acquires merit for himself. Of him it is written (Mishlei 6:30) "they will not shame a thief for stealing (to fill his soul that is hungry" [for Torah].) In the end, he will be appointed (as a teacher) over the congregation (and he will return the "theft" in full.) As it is written (Ibid. 31) "In the end he will repay seven-fold. All the wealth of his house will he give" — "seven-fold" signifying Torah, viz. (Psalms 12:7) "The words of the L–rd are pure words, silver purged in an earthen crucible, refined seven-fold."