הלכה על סנהדרין 40:11

And this is from the commandments that are incumbent upon the entire community; and like the matter that they, may their memory be blessed, said (Sanhedrin 20b), "Three commandments were commanded to Israel at the time of their entrance to the land: to appoint themselves a king; to build themselves the Choice House (the Temple); and to cut off the seed of Amalek." And in truth is it that the obligation to kill them and destroy them from the world is also incumbent upon every male individual from Israel - if they have the power in their hands - in every place and at all times, if he finds one from all of their seed. And one who transgresses this and one from the seed of Amalek [chances upon] him - and he has the wherewithal in his hand to kill him - and he does not kill him, he has violated this positive commandment.