Question: Many people customarily wash their hands and then recite the “Al Netilat Yadayim” blessing as they are drying their hands. Is this custom halachically sound?
Answer: The Gemara (Pesachim 7b) teaches us a great rule which is that one must recite the blessing on a given Mitzvah before performing that Mitzvah, besides for the Mitzvah of the immersion of a convert. For instance, before eating Matzah, one recites the blessing of “Al Achilat Matzah” and before immersing new vessels into the Mikveh, one recites the blessing of “Al Tevilat Kelim.” The only exception to this rule is when a non-Jew converts to Judaism and cannot recite a blessing before he immerses in the Mikveh, for he cannot exclaim, “Asher Kideshanu Be’Mitzvotav Ve’Tzivanu” (“Who has sanctified us with his Mitzvot and commanded us) since no new sanctity rests upon him yet. Our Sages therefore enacted that the convert recite this blessing after he has immersed in the Mikveh and become a Jew.
Nevertheless, regarding all other Mitzvot without exception (besides for continuing Mitzvot), one must recite the blessing on the Mitzvah before performing the Mitzvah. If one recites a blessing after performing any given Mitzvah, this is considered a blessing in vain. In such a case, according to the Rambam, one transgresses the negative Torah commandment of “Do not utter the name of Hashem, your G-d, in vain.”
It would seem that one must likewise recite the “Al Netilat Yadayim” blessing before performing the Mitzvah of washing one’s hands.
There are, nonetheless, several different opinions about when to recite the “Al Netilat Yadayim” blessing. According to Rabbeinu Chananel, the Or Zarua, and other Rishonim, one should recite this blessing immediately prior to beginning the hand-washing in order to recite the blessing before performing the Mitzvah. We do not customarily follow this opinion, however.
On the other hand, other Rishonim (quoted by the Or Zarua, Chapter 79) are of the opinion that the “Al Netilat Yadayim” blessing need not be recited before the hand-washing, for there are times when one’s hands are unclean. Thus, our Sages never enacted that one must recite the blessing before washing one’s hands and one may recite the blessing even after washing one’s hands. The Halacha does not follow this opinion either.
The intermediate opinion is that of the Tosafot and many other Poskim who rule that one should recite this blessing after washing one’s hands, when one’s hands are clean, but before drying one’s hands. The reason for this is because drying one’s hands is also part of the Mitzvah, for one must dry his hands before eating. Thus, one will have recited the blessing when his hands are clean but while one is still in the middle of performing the Mitzvah.
The custom among the Jewish nation is in accordance with this last opinion, i.e. to recite the blessing after washing one’s hands but before drying them. According to Maran Ha’Shulchan Aruch (Chapter 158, Section 1), if one recites the blessing after one has dried his hands, this is a blessing in vain. Indeed, the Rambam and most Rishonim maintain that if one forgets to recite the blessing before drying one’s hands, one may not recite it after drying them, for doing so would entail reciting a blessing in vain.
Summary: One should recite the “Al Netilat Yadayim” blessing after washing one’s hands but before drying them. If one did not recite the blessing and has already dried one’s hands, one should not recite the blessing at all. Maran Rabbeinu Ovadia Yosef zt”l rules likewise in his Halichot Olam (Volume 1, page 317).