Both Siddur Vilna and Artscroll give the source as a Targum Yerushalmi on the verse of "Veyichal Elokim" (Gen:2:2) which the targum renders as "God longed" - Artscroll adds this is also the explanation given by Abudraham.
I would add that the Targum on Shmuel II 13:39 on the verse "Va'tchal Dovid" also translates the word as meaning "chemdah" (see Rashi, Radak and Metzudos there). Actually, in Shmuel the translation of "chemdah" would seem to be literally the pshat while in Beraishit is most likely the drash.
Tzvi Daum
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