Musar על עירובין 43:2

And it is necessary to be cruel in judgment in order not to favor relatives, friends and the poor, but to decide according to the law. And our Rabbis said further: "In whom do you find the words of the Torah? In him who makes himself as cruel as the raven to his children, as Rabbi Adda bar Matna who was going to the House of Study and his wife said to him, 'What shall I do for your children' (to feed them if you go to study Torah instead of to work)? He responded, 'There are vegetables in the swamp' " (Erubin 22a).