And the 44th mitzvah is that of the Omer1 grain offering. This is the commandment to bring an offering of barley on the 16th of Nissan, together with a year-old sheep as a burnt-offering.

The source of this commandment is G‑d's statement2 (exalted be He), "You must bring an Omer of your first reaping to the priest."

This grain offering is called minchas bikkurim [first grain offering], as hinted to in G‑d's statement3 (exalted be He), "When (im) you bring the first grain offering to G‑d, [it should be brought] as soon as it ripens; fresh kernels which are roasted in fire and ground up."

The Mechilta4 says, "The word 'im' [literally, "if"] when used in Scripture generally means something optional, except for three exceptions which are obligatory." One of them is G‑d's statement, "When ["im"] you bring the first grain offering to G‑d." Our Sages said, "'It is an obligation.' 'You say it's an obligation, but you might think it's really optional!' The Torah therefore continues,5 'You shall bring your first grain offering.' This clearly refers to an obligation, not something optional."

All the details of this mitzvah have been fully explained in the 10th chapter of Menachos.6