Midrash su Salmi 118:18

R. Yishmael says: A Canaanite man-servant can never be redeemed and can go free only by consent (of the master), as it is written (Leviticus 25:46) "And you shall hold them as an inheritance … to inherit as a holding, etc." And in our manner we have learned that a Canaanite man-servant is like a field of holding forever. But if his master persecuted him and knocked out his tooth or blinded his eye or (maimed) one of his (other) external organ prominences, he acquires himself (i.e., his freedom) with afflictions. Now does this not follow a fortiori, viz.: If one can acquire himself by afflictions from the hand of flesh and blood, how much more so, from the hands of Heaven! And thus is it written (Psalms 118:18) "The L rd has afflicted me exceedingly and He did not consign me to death."