A cappella:

While worship in the Temple in Jerusalem included musical instruments (2 Chronicles 29:25–27), traditional Jewish religious services in the Synagogue, both before and after the last destruction of the Temple, did not include musical instruments given the practice of scriptural cantillation. The use of musical instruments is traditionally forbidden on the Sabbath out of concern that players would be tempted to repair (or tune) their instruments, which is forbidden on those days. (This prohibition has been relaxed in many Reform and some Conservative congregations.) Similarly, when Jewish families and larger groups sing traditional Sabbath songs known as zemirot outside the context of formal religious services, they usually do so a cappella, and Bar and Bat Mitzvah celebrations on the Sabbath sometimes feature entertainment by a cappella ensembles. During the Three Weeks musical instruments are prohibited. Many Jews consider a portion of the 49-day period of the counting of the omer between Passover and Shavuot to be a time of semi-mourning and instrumental music is not allowed during that time. This has led to a tradition of a cappella singing sometimes known as sefirah music.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What is forbidden on the Sabbath because it was considered work?
unanswerable