Halakhah do Samuela I 15:2

That is that He commanded us to remember what Amalek did to us in its coming forward to do us evil, to hate it at all times, to arouse the spirits with statements to fight with it and to make the nation alacritous to hate it - such that the commandment not be forgotten with [the passing of] so much time. And that is His, may He be exalted, saying, "Remember what Amalek did to you" (Deuteronomy 25:17). Perhaps only in your heart? When it states, "you shall not forget," behold that is forgetting of the heart. So what do I establish [with] "Remember?" That hatred for it be in your mouth. Do you not see - that at the beginning of Shmuel's doing this commandment - how he first mentioned its evil acts and [only] then did he command to kill them? And that is His, may He be blessed, saying, "I have remembered what Amalek did to Israel" (I Samuel 15:2). (See Parashat Ki Tetzei; Mishneh Torah, Kings and Wars 5.)