R. Eliezer says: The L–rd swears by His throne of glory: If there comes a man of all the nations to be proselytized, he will be accepted; but Amalek and his household will not be accepted, viz. (II Samuel 1:13) "And David asked the youth who told him: Where are you from? And he said: I am the son of an Amalekite convert." David remembered at that time what had been told to Moses our teacher, that if there comes a man of all the nations of the world to be proselytized, he should be accepted, but if he is of the house of Amalek, he should not be accepted, whereupon (II Samuel , Ibid.) "David said to him: Your blood is on your own head, for your mouth testified against you." This is the intent of (Exodus 17:17) "from generation to generation."