מדרש על ויקרא 24:20

(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.