Musar על כתובות 134:6

He who gives alms to the poor grudgingly loses the merit of the deed, even though he gives much, and it is better that he give only one pruta with a pleasing countenance. And it is well to give before one is asked, and it is well to give secretly, as it is written, "A gift in secret pacifieth anger" (Prov. 21:14). There were people who used to tie the money in a garment and cast it behind them and the poor would come and take, so that he who gave the alms did not know to whom he gave, and he who received the alms did not know from whom he received it, and thus the poor man was not shamed. The general rule in this matter is that as much as possible should be concealed, so that the poor man does not know who is the giver and the giver does not know who is the recipient.