(Devarim 16:21) "You shall not plant for yourself an asheirah (a tree devoted to idolatry)": We are hereby taught that one who plants an asheirah transgresses a negative commandment. And whence is it derived that one who plants a tree in the Temple mount transgresses a negative commandment? From "You shall not plant for yourself … any tree beside the altar of the L-rd your G-d." R. Eliezer b. Yaakov says: Whence is it derived that a portico is not built in the azarah (the Temple court)? From "any tree (i.e., wood) beside the altar of the L-rd your G-d." "that you make for yourself": to include (in the interdict planting a tree beside) a bamah (a temporary altar).
Sifrei Devarim 145
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