Deuteronomy 25:17-19

17Remember what Amalek did unto thee by the way, when ye were come forth out of Egypt; 18How he met thee by the way, and smote the hindmost of thee, even all that were feeble behind thee, when thou wast faint and weary; and he feared not God. 19Therefore it shall be, when Yhwh thy God hath given thee rest from all thine enemies round about, in the land which Yhwh thy God giveth thee for an inheritance to possess it, that thou shalt blot out the remembrance of Amalek from under heaven; thou shalt not forget it.