Halakhah zu Wajikra 2:14

That is that He commanded to offer the grain offering of the omer. And that is the grain offering of barley on the sixteenth day of Nissan. And with it, we offer an unblemished year-old lamb. And that is His, may He be exalted, saying, "you shall bring the omer, etc." (Leviticus 23:10). This grain offering is what is called, "first fruits." And He hinted to it in His, may His name be blessed, saying, "And if you offer an offering of first fruits" (Leviticus 2:14). And the language of the Mekhilta (Mekhilta d'Rabbi Yishmael 20:22:1) is, "Every, if, in the Torah connotes optionality, except for three which are obligatory. One is, 'And if you offer an offering of first fruits.' You say it is obligatory, but perhaps it is optional. [Hence] we learn to say, 'You shall offer the offering of your first fruits.' It is obligatory, not optional." And the regulations of this commandment have already all been completely explained in the tenth chapter of Menachot. (See Parashat Emor; Mishneh Torah, Daily Offerings and Additional Offerings 17.)

The commandment of the sacrifice of the omer of barley: That we offer on the second day of Pesach, beyond the additional offering of the rest of the days of Pesach, a one-year old sheep for a burnt-offering and one omer of barley, that is called the omer of waving - as it is stated (Leviticus 23:10-11), "When you come to the land, etc., you shall bring the omer, the beginning of your harvest, etc. And he shall wave the omer in front of the Lord from the morrow of the Shabbat." And Onkelos translates, "after the holiday" - meaning to say, on the second day of Pesach. As behold, it is referring to Pesach in the section before this. And it is stated there (Leviticus 23:12), "And you shall make on the day of your waving the omer an unblemished one-year old sheep, etc." And this sacrifice of the omer is called the offering of the first fruits. And it is a hint to this when He may He be blessed, says (Leviticus 2:14), "And if you shall bring an offering of the first-fruits to the Lord, new roasted with fire, etc." And the language of Mekhilta d'Rabbi Yishmael 22:24 is "Each and every 'if' in the Torah is optional, etc. except for three that are obligatory and this is one of them." And they said there, "You say it is an obligation or is it only optional? [Hence] we learn to say, 'you shall offer the offering of your first fruits' - [it is] an obligation and not optional. And the matter of the offering is thus (Menachot 63b): That they would bring three seah of barley, and they would take out one issaron from all of it, which they would sift with thirteen sieves. And the rest would be redeemed and eaten by any person. And it is liable for the hallah-tithe but exempted from the [other] tithes. And we take this issaron of fine barley flour and we mix it with a log of oil and place a handful of frankincense upon it - like the other meal-offerings. And the priest waves it in the East - he extends [it] and brings [it back], raises [it] and lowers [it] - and presents it across from the point of the southwest corner, like the other meal offerings. And he takes a handful and incinerates [it], and the rest is eaten by the priests, like the remainders of all of the meal-offerings (Menachot 67b).