Reference for Makkot 16:13

לרבנן דאמרי סייף חמור מחנק הורג אביו [בשוגג] שגגת סייף הוא ושגגת סייף ניתנה לכפרה

For the Rabbis who say beheading is more severe than strangulation: one who inadvertently kills his father, this is inadvertent sin [that would be punished, if done intentionally, with] beheading, so such an inadvertent sin would be given atonement [through exile—so this is the opinion of our Mishnah].

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