TT #397 · Parashas Reeh · Seventh Aliyah · Passuk 16:9
Seven weeks you shall count for yourself; from the time the sickle is first put to the standing grain you shall begin to count seven weeks.
Torah Temimah points to Menachos.
English: "From the time the sickle is first put to the standing grain" — It was taught: One might have thought that one may reap and bring [the Omer] and count whenever one wishes; therefore the verse says "from the time the sickle is first put to the standing grain you shall begin to count."
Derash on the verse (Menachos) — the count begins on the day of the reaping.
The counting of the Omer begins on the day the first sheaf is reaped.