Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
First Section – the Positive Commandments
30. It is a positive commandment to hear the sounding of the Shofar on Rosh HaShana.
As it states in Bamidbar (Numbers) 29:1,"It is a day of blowing (the Shofar) for you." A shofar is a sheep's horn that is curved and other shofars are not acceptable. A person is commanded to hear nine blasts of the shofar on Rosh HaShana: three times Tekiah (one long blast), Tru'ah (numerous short blasts), and Tekiah. We are in doubt what Tru'ah is, therefore we blow three times Tekiah, Shevarim-Tru'ah, Tekiah; three times Tekiah, Shevarim, Tekiah; and three times Tekiah, Tru'ah, Tekiah so we can be free from any doubt.
This is obligated in all places and at all times, for men but not for women.