There is a frequently cited midrash that we read "Guard the Matzos" as "Guard the Mitzvos". I see this cited all over the internet, but no one seems to give the exact source. Could someone let me know what it is?
Relatedly, I've seen a citation to Mekhilta Shemos 12 for the statement "When a Mitzvah comes into your hand, don't let it become chametz." Is the Mekhilta the source for both statements? And does anyone know more specifically where in Ch. 12 it appears?