Deuteronomy 25:17-19 -

Whilst in their intercourse with each other the law of love and brotherly kindness was to predominate, it was to be otherwise in regard to the enemies of God and his people. Them they were to overcome by force; wickedness was to be removed by the extinction of the wicked. Moses has already repeatedly reminded the Israelites that they had utterly to destroy the wicked nations of Canaan; and he here closes this discourse by reminding them that there was a nation outside of Canaan which was also doomed, and which they were to root out. This was Amalek, which had attacked the Israelites in their journey at Rephidim, and had taken advantage of their exhausted condition to harass their rear and destroy those who, faint and weary, had lagged behind. For this they had been already punished by the Israelites, who, led on by Joshua, had turned upon them and discomfited them with the edge of the sword. This, however, was not enough; Amalek was to be utterly destroyed, and this the Israelites were to effect as soon as the Lord had given them rest in the Promised Land. It was not, however, till the time of David that this was done.