Reference for Berakhot 8:2
בֵּית שַׁמַּאי אוֹמְרִים, נוֹטְלִין לַיָּדַיִם, וְאַחַר כָּךְ מוֹזְגִין אֶת הַכּוֹס. וּבֵית הִלֵּל אוֹמְרִים, מוֹזְגִין אֶת הַכּוֹס וְאַחַר כָּךְ נוֹטְלִין לַיָּדָיִם:
Beth Shammai say: First the hands are washed and then the wine cup is poured. [For if you say that first the wine cup is poured, perhaps drops will fall on the outside of the cup and become tame (ritually unclean) because of his hands. For hands, before they are washed, are of second-order tumah (uncleanliness), and they impart first-order tumah to liquids, which, in turn, render the outside of the cup tame. For liquids which have become tame impart tumah to vessels by rabbinical ordinance. They were lenient in respect to this tumah in that if the outside of a vessel became tame through unclean liquids, neither its inside, nor his hands, nor its rim become tame. And Beth Shammai hold that it is forbidden to make use of a vessel whose outside is tame even though its inside has not become tame — a decree lest drops spurt from within it to the outside of the cup and the drops become tame because of the outside and in turn impart tumah to his hands. And because it is forbidden to use a vessel whose outside is tame, first the hands are washed, and then the wine is poured, so that the drops on the outside not become tame through the hands and render the outside of the cup tame and he use the cup illicitly.] And Beth Hillel say: First the cup is poured and then the hands are washed. [Beth Hillel hold that it is not forbidden to use a vessel whose outside is tame. Therefore, first he pours the cup and drinks it, and then he washes his hands. For if you say that he washes first and then pours the cup, it may be that the outside of the cup is tame, it being permitted to use a vessel whose outside is tame, and it may be that his hands are not thoroughly dried, so that the outside of the vessel imparts tumah to the water on his hands, and that water, having become first-order tumah, in turn makes his hands tame, and he eats with "sullied" hands.]