Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
First Section – the Positive Commandments
77. It is a positive commandment to destroy any offspring of Amalek
As it states in Devarim (Deuteronomy) 25:19, "Erase the memory of Amalek." Today, we are not sure who is part of Amalek until Eliyahu (Elijah) the Prophet comes and tells us who they are. Then we will be able to wipe out Amalek's memory from under Heaven. May G-d let us see the coming of Eliyahu the Prophet and the righteous Moshiach (Messiah) speedily in our days.
This concludes all of the positive commandments that apply during our days. Their count is 77, which in Gematria equals "You shall live by them" [each Hebrew letter has a number. By adding the letters in the statement 'you shall live by them' together, you get 77.]