"Since Zevulun and Issachar were partners, Issachar was engaged in Torah study and Zevulun was engaged in commerce, and Zevulun would toil and give to the mouth of Issachar... 
That the deed is greater than the doer, for without Zevulun, Issachar would not have been able to engage in the study of the Torah, as Zevulun would feed him and put food into his mouth... 
Zevulun and Issachar both received the reward for Torah study together, and both earned their livelihood together... Just as Zevulun had a share in the reward of his Torah, so too Issachar had a share in Zevulun's wealth."
– Midrash Rabbah, Parashat Yitro, 17.