(Genesis 1:9) "And God said: Let the waters be gathered [yikkavu]" — [meaning] let a measure [middah] be made for the waters, just as you say, "and a line [kav] shall be stretched over Jerusalem" (Zechariah 1:16).
A Measuring Line Stretched Over the Waters
Yalkut Shimoni on Torah 7:1
(בראשית א ט) וַיֹּאמֶר אֱלֹהִים יִקָּווּ הַמַּיִם, יֵעָשֶׂה מִדָּה לַמַּיִם כְּהַאֵיךְ דְאַתְּ אָמַר (זכריה א, טז) "וְקָו יִנָּטֶה עַל יְרוּשָׁלָיִם".