Shulchan Arukh, Yoreh De'ah 246

REMA: A person should not think to engage in Torah and to acquire wealth or honor with the learning, for one who ventures to think such a thought will not achieve the crown of Torah. Rather, one should make his Torah fixed and his work contingent, and should reduce business and engage in Torah. He should put aside temporary pleasures from his mind and do work all day for his livelihood if he does not have enough to eat, and the rest of the day and the night he should engage in Torah. And it is a great quality to generate one's sustenance from the work of his hands, as it says, "The product of your hands you shall surely eat" (Ps. 128). Anyone who puts in his mind occupying himself with Torah and not working, but supporting himself from tz'dakah, behold, this one desecrates the Divine name and dishonors the Torah. For it is forbidden to derive benefit from words of Torah, and any Torah that does not have work along with it, it attracts sin and he winds up robbing people. And all of this applies to one who is healthy and can engage in his work or in a profession a little and provide for his livelihood. However, an elder or a sick person is permitted to derive benefit from his Torah such that they will provide for him. And some say that it is permitted even for a healthy person.

And therefore the practice in all Jewish communities has been that the Rav of the city has an income and provisions from the other people of the city, so that he will not have to engage in work in front of other people, so that the Torah becomes degraded in front of the multitude. And [this applies] specifically to the sage who needs it, but for a wealthy person it is prohibited. And some are more lenient, saying that it is permitted for a sage and his students to accept subsidies from those who donate in order to strengthen the hands of those who study Torah, since in this manner they can engage in Torah in affluence. Nevertheless, one who is able to provide for himself well from the work of his own hands and to engage in Torah, it is a pious quality and a gift of God, but this is not the nature of all persons, for it is impossible for everyone to engage in Torah and to become wise in it and to provide for himself by himself....