The commandment for counting the Omer is recorded within the Torah in Leviticus 23:9–21

When ye are come into the land which I give unto you, and shall reap the harvest thereof, then ye shall bring the sheaf (omer) of the first-fruits of your harvest unto the priest. And he shall wave the sheaf before the LORD, to be accepted for you; on the morrow after the day of rest the priest shall wave it. ... And ye shall count unto you from the morrow after the day of rest, from the day that ye brought the sheaf of the waving; seven weeks shall there be complete; even unto the morrow after the seventh Shabbat shall ye number fifty days; and ye shall present a new meal-offering unto the LORD. ... And ye shall make proclamation on the selfsame day; there shall be a holy convocation unto you; ye shall do no manner of servile work; it is a statute for ever in all your dwellings throughout your generations.