C. F. Keil and Franz Delitzsch on Exodus 17:14

Exo 17:14

As this battle and victory were of such significance, Moses was to write it for a memorial בּסּפר, in “the book” appointed for a record of the wonderful works of God, and “to put it into the ears of Joshua,” i.e., to make known to him, and impress upon him, that Jehovah would utterly put out the remembrance of Amalek from under heaven; not “in order that he might carry out this decree of God on the conquest of Canaan, as Knobel supposes, but to strengthen his confidence in the help of the Lord against all the enemies of Israel. In Deu 25:19 the Israelites are commanded to exterminate Amalek, when God should have given them rest in the land of Canaan from all their enemies round about.