Parshas Shemos

(The Segulah of Learning Zera Shimshon)

The pasuk tells us that Moshe Rabbeinu was on the way to the inn and Hashem sought to kill him. Rashi explains that this was because Moshe had not yet given his son Eliezer a bris milah.

The Zera Shimshon says that this is hard to understand, because certainly someone who neglects to himself have a bris milah is punishable by kares, but not giving his son a bris was "just" a failure to perform a mitzvas aseh and not punishable by death!

Bereishis Rabba (49:2) tells us that Avraham Avinu merited to have Moshe Rabbeinu as a descendant because he gave himself a bris. Moshe Rabbeinu's job was not only to save Bnei Yisroel from Mitzrayim, but to ensure they all had a bris milah as well.

Moshe was born in the merit of bris milah, and it was incumbent upon him to make sure that every member of klal yisroel received a bris, therefore it was punishable by death for Moshe to put off giving his own son a bris milah. (The pasuk writes "lamulos," "cirmcumcisions," in plural to tell us that he was responsible for the whole nation to do this mitzvah of bris milah.)

Source: Zera Shimshon Parshas Shemos