Rambam, Mishneh Torah, Laws of Gifts to the Poor 10:8

Below this is one who gives tzedakah to the poor, and the giver does not know to whom s/he gives nor does the recipient know from whom s/he takes. For this is an example of doing a mitzvah for its own sake [and not for any benefit] For example: there was a room in the Temple where the righteous would secretly place their donations, and the poor would secretly take these donations. Close to this is the one who places money in a tzedakah box. But one should not put money into a tzedakah box unless one knows that the person charged with distributing the money is as trustworthy and wise and as meritorious in action as Rabbi Hananiah ben Tradion. (a Talmudic tzedakah collector known for his integrity). [Translation by Rabbi Jill Jacobs]