Бамидбар 29:1 Галаха: Mishneh Torah, Shofar, Sukkah and Lulav, Mishneh Torah, Prayer and the Priestly Blessing, The Sabbath Epistle & Mishneh Torah, Sabbath

It is a positive commandment from the Torah to hear the sound of the shofar (animal horn) on Rosh Hashanah; as it is stated (Numbers 29:1), "it shall be a day of blowing for you." The shofar which is blown — both on Rosh Hashanah and for the Jubilee year — is a bent ram's horn. And all shofars are disqualified except for the horn of a ram. Even though blowing with a shofar is not mentioned explicitly [in the passage] about Rosh Hashanah; it surely does state about the Jubilee year (Leviticus 25:9), "you shall blast the shofar." And we have learned through the oral tradition that just as the blowing of the Jubilee year is with a shofar, so too is the blowing of Rosh Hashanah with a shofar.

We do not blow with a shofar of idolaters at the outset. But if one did blow [with one], he has fulfilled [the commandment]. And if one blew [with a shofar] of a condemned city (eer hanidachat), he has not fulfilled [his obligation. Regarding] a stolen shofar with which he has blown, he has fulfilled [the obligation]. For the commandment is only listening to the sound — even though he did not touch it and did not raise it up, the listener has fulfilled [it] — and there is no law of theft with sound. And likewise should one not blow with the shofar of a burnt offering; but if he did blow [with it], he has fulfilled [the obligation]. For there is no law of misappropriating [the sacred] with sound. And if you say, "Did he not derive pleasure from listening to the sound"; [the answer is] that the commandments are not given for pleasure. Hence, one who has vowed not to derive pleasure from the shofar is permitted to blow the blow of the commandment with it.

How many blasts is one obligated to hear on Rosh Hashanah? Nine blasts. Since it is stated, "teruah," with regards to the jubilee year and to Rosh Hashanah three times. And every teruah requires a simple blast (tekiah) before it and a simple blast after it. And from the oral tradition, they learned that all of the teruahs of the seventh month are the same: Whether on Rosh Hashanah, or whether on Yom Kippur of the Jubilee year, we blow nine blasts on both of them — tekiah, teruah, tekiah; tekiah, teruah, tekiah; tekiah, teruah, tekiah.