Midrash su Geremia 45:3

Issachar is a large-boned ass (Gen. 49:14). That is, he bears the yoke of the Torah like an ass that bears its load, while Zebulun brings his wares in ships. Crouching down between the sheepfolds (ibid.). This refers to his disciples, who spread the knowledge of the Torah before the wise throughout the land, as it is said: When ye lie among the sheepfolds, the wings of the dove are covered with silver (Ps. 68:14). For he saw a resting place that it was good (Gen. 49:15). This refers to the Torah, as it is said: I am weary with my groaning, and I find no rest (Jer. 45:3). That it was good alludes to the Torah, since it is said: For I give you good doctrine (Prov. 4:2). And he bowed his shoulders to bear (Gen. 49:15) the yoke of the Torah. And he became a servant under task-work (ibid.). Task-work refers to the law. Whenever the people erred in the law, they would seek a decision from him, as is said: And the princes of Issachar were with Deborah; as was Issachar, so was Barak; into the depth they rushed forth at his feet (Judg. 5:15); for he would search deeply into the law.