Parashas Behar · Aliyah 1 · Passuk 25:09
Chabad.org — English translation + Rashi
Torah Temimah cites Rosh Hashanah 33b.
English: “And you shall sound a shofar.” The Rabbis taught: From where do we know that it is with a shofar? The verse says “and you shall sound a shofar.” From where a plain [tekiah] before it? The verse says “and you shall sound a shofar of teruah.” From where a plain after it? The verse says “you shall sound a shofar.” From where three of three of three? The verse says “and you shall sound a shofar of teruah.”
Classical derasha (Bavli Rosh Hashanah).
The required sequence of tekiah-teruah-tekiah and the total of nine blasts is derived from the multiple occurrences of the shofar-teruah language.