Le Deutéronome 17:11 Halakha : Abudarham, Mishneh Torah, Blessings, Mishneh Torah, Sabbath & Mishneh Torah, Sanctification

...One must recite a blessing before fulfilling any commandment, whether biblical or rabbinic, whether unconditional or conditional. (Examples of biblical commandments that are unconditional include tefillin, tzitzis, shofar, sukkah, and lulav. Examples of biblical commandments that are conditional include mezuzah and maakeh. Examples of rabbinic commandments that are unconditional include lighting shabbos candles, lighting the menorah, and reading the megillah. Examples of rabbinic commandments that are conditional include eruv and netilas yadayim.) But why are rabbinic commandments treated like biblical commandments? Scripture states: "According to the law that they will teach you, and according to the ordinance that they will tell you, do not deviate from what they say to you at all!" In this verse, we are commanded to obey the rabbis. Therefore, we recite a blessing before fulfilling rabbinic commandments.

When washing the hands, whether for a meal or for the recital of the Shema or the Amidah prayer, one first says the blessing "Who hast sanctified us by Thy commandments and given us command concerning the washing of the hands". For this is an ordinance of the sages whose instructions the Torah bids us heed, as it is said "According to the law which they shall teach thee' (Deut. 17-11). The washing of the hands after a meal requires no blessing, since it was only instituted as a precautionary measure against danger to health.^*that might arise from touching the eyes with fingers that have salt adhering to them. (See next paragraph.) Special attention should, on this account, be paid to the observance.

One who kills creeping things that are fruitful and multiply from a male and a female, or come to be from the dirt – such as fleas – is liable like one who kills a beast or an animal. But one who kills creeping things that come to be from dung or from rotten produce and that which is similar to them – such as meat worms and worms in legumes – is exempt.