Translation:Mikraot Gedolot/Deuteronomy/22:7

Text
You shall send away the mother, and the children you may take to yourself; so that it shall be good for you, and you shall prolong [your] days.

Rashi
So that it shall be good for you

(Ibid. 142): If a light mitzvah, that does not involve a loss of money, the Torah said: ”so that it shall be good for you, and you shall prolong [your] days”, it is an argumentum a fortiori for the giving of reward of grave mitzvoth.