Yosef Karo, Shulchan Aruch — Yoreh Deah 18 The law of checking the knife and its flaw
“ If perhaps he reversed the direction, the slaughter is unacceptable. And one who cuts and doesn't feel the flaw before the slaughter, only if we found his knife acceptable before the slaughter; however (now) after the slaughter we find a disputable flaw, as it is said: it is clear me that if he cut in only one direction, the direction without flaw alone, the slaughter is unacceptable.סכין שתבדק בהולכה, ולא הרגיש שיש בה פגם, וכשהחזיר אותה בהבאה הרגיש שיש בה פגם – זו הנקראת "מסוכסכת", אם שחט בה דרך הולכה ולא הביא – שחיטתו כשרה. ”
