Alongside the Torah (the Pentateuch), which is the written law of the Jewish People,  there developed an oral tradition of interpretation. This tradition was put down in writing and sealed at the beginning of the third century c.e. by Rabbi Yehudah Ha-Nassi, the head of the Jewish Assembly, since it was feared at that time that the tradition would be lost. The Mishnah is considered to be a source of religious legal authority second only to the written Torah. 

photograph: A paragraph from the Mishnah, preserved in the Cairo Genizah: "Rabbi Yohanan said: Whoever fulfills the Torah despite poverty, will ultimately fulfill it in wealth; but whoever neglects the Torah because of wealth, will ultimately neglect it in poverty."