Jews are meant to be workers according to the Torah, or there would not be so many of the halachos relating to business practices and farming. However we also see the fact that Zevulun, the businessman, according to his father Yaakov Avinu, is meant to support Yissachar in his learning. So that is IMO the primary source for this. (It does not however, say Yissachar CANNOT take a job. You wanted a source, though).