מדרש על סנהדרין 39:18

(Devarim 12:10) "And you will cross the Jordan and you will dwell in the land … (11) and the place that the L-rd your G-d will choose, etc." R. Yehudah says: Three mitzvoth were commanded to Israel upon their entrance to the land: to appoint a king over them, to build the Temple, and to cut off the seed of Amalek,