Vayikra
Leviticus 23:15-16
The Torah commands counting 'sheva shabbatot temimot' — seven complete weeks — from the day of the Omer offering until Shavuot, and only 'on the morrow of the seventh week' do you bring the new offering (the two loaves).
וּסְפַרְתֶּ֤ם לָכֶם֙ מִמׇּחֳרַ֣ת הַשַּׁבָּ֔ת מִיּוֹם֙ הֲבִ֣יאֲכֶ֔ם אֶת־עֹ֖מֶר הַתְּנוּפָ֑ה שֶׁ֥בַע שַׁבָּת֖וֹת תְּמִימֹ֥ת תִּהְיֶֽינָה׃ עַ֣ד מִֽמׇּחֳרַ֤ת הַשַּׁבָּת֙ הַשְּׁבִיעִ֔ת תִּסְפְּר֖וּ חֲמִשִּׁ֣ים י֑וֹם וְהִקְרַבְתֶּ֛ם מִנְחָ֥ה חֲדָשָׁ֖ה לַיהֹוָֽה׃
And from the day on which you bring the sheaf of elevation offering—the day after the sabbath—you shall count off seven weeks. They must be complete: you must count until the day after the seventh week—fifty days; then you shall bring an offering of new grain to GOD.
Why it matters — The word 'temimot' (complete/perfect) is the biblical basis for the requirement that each day of the Omer be whole, which underlies the prohibition of beginning Shavuot early.