Midrash su Levitico 18:12

4) (Vayikra 21:2) ("Only for his flesh that is near to him, his mother and his father, his son and his daughter and his brother.") "Only for his flesh that is near to him": "flesh" is his wife, viz. (Vayikra 18:12) "She is the flesh (i.e., the wife) of your father." "that is ner": to exclude one that was betrothed to him. "to him": to exclude one that he divorced. Let it be written (only) "to his mother." Why need "to his father" be written? (i.e., it can be derived a fortiori), viz.: If he makes himself tamei to his mother, who becomes a challalah (by relations with one who is unfit), how much more so should he make himself tamei to his father, who does not make himself a challal thereby! — If so, I would say: Just as his mother is definitely known (to be his mother), so his father must be definitely known. Whence would I know (that he makes himself tamei for him even when he is known as) his father by common acceptance? It must, therefore, be written "to his father."