Rashi on Deuteronomy 16:9/רש"י על דברים ט״ז:ט׳

1מהחל חרמש בקמה [BEGIN TO NUMBER SEVEN WEEKS] FROM THE TIME THOU BEGINNEST TO PUT THE SICKLE TO THE CORN — i.e. from when the “Omer” has been cut (from the sixteenth of Nisan) which is the first produce to be harvested (Leviticus 23:10) (cf. Sifrei Devarim 136:2-3; Menachot 71a).