Midras Keluaran 21:15: Mekhilta DeRabbi Yishmael, Sifra, Bamidbar Rabbah & Shemot Rabbah

(Exodus 21:15) "And if one strikes his father and his mother, he shall be put to death": What is the intent of this? Because it is written (Leviticus 24:20) "an eye for an eye, etc.", and Scripture took it (striking father or mother) out of its category to make it more stringent, to make it liable to the death penalty, this section was, therefore, stated.

7) But it is impossible to write of striking vis-à-vis the L–rd, (as it is written vis-à-vis parents [Shemoth 21:15]). (And one's honoring parents) is justly (compared to his honoring the L–rd), for the three of them are partners in his creation.

8) "one who strikes a man": What is the intent of this? Because it is written (Shemoth 21:15) "And one who strikes his father and mother (shall be put to death"), I might think that he is not liable until he strikes both of them together; it is, therefore, written "and one who strikes a man shall be put to death" — even one of them. I might think that even if he struck them and did not cause an injury he is liable; it is, therefore, written "And one who strikes a beast … and one who strikes a man." Just as one who strikes a beast (is not liable) until he causes an injury, so, he who strikes a man. I might think that even if he struck them (his father or his mother) after (their) death, he is liable, (just as he is if he curses them after their death); it is, therefore, written "one who strikes a beast" and "one who strikes a man" — Just as the first, while it is living, so, the second, while he is living.