Positive commandment · Mitzvat Aseh #161 of 613
To count 49 days from the harvesting of the omer
לספור מקצירת העומר תשעה וארבעים יום
The verse · Vayikra 23:15
וּסְפַרְתֶּם לָכֶם מִמׇּחֳרַת הַשַּׁבָּת מִיּוֹם הֲבִיאֲכֶם אֶת־עֹמֶר הַתְּנוּפָה שֶׁבַע שַׁבָּתוֹת תְּמִימֹת תִּהְיֶינָה
“And you shall count for yourselves from the morrow after the sabbath, from the day that you brought the ῾omer of the wave offering; seven complete sabbaths shall there be:”
The Rambam’s words
To count 49 days from the harvesting of the omer, as [Leviticus 23:15] states: "And you shall count from the day following the day of rest...."
לספור מקצירת העומר תשעה וארבעים יום, שנאמר: "וספרתם לכם, ממחרת השבת" (ויקרא כג,טו).
Why · Sefer HaChinuch #306
Counting the omer
“It is from the roots of the commandment from the angle of the simple understanding [that it is] since the entire essence of Israel is only the Torah, and because of the Torah were the heavens and earth created, and as it is stated (Jeremiah 33:25), “Were it not for my covenant day and night, etc.””
Who & when
“And this commandment of counting the omer is practiced from Torah writ in every place by males at the time of the [Temple], such that the omer is there; and rabbinically in all places, even when the omer is not offered.”
Where in the Mishneh Torah
Hilchot Daily Offerings and Additional Offerings הלכות תמידים ומוספין