Positive commandment · Mitzvat Aseh #24 of 613
For a priest to sanctify his hands and feet at the time of service [in the Temple]
לְקַדֵּשׁ הַכּוֹהֵן יָדָיו וְרַגְלָיו בִּשְׁעַת הָעֲבוֹדָה
The verse · Shemot 30:19
וְרָחֲצוּ אַהֲרֹן וּבָנָיו מִמֶּנּוּ אֶת־יְדֵיהֶם וְאֶת־רַגְלֵיהֶם
“For Aharon and his sons shall wash their hands and their feet thereat:”
The Rambam’s words
For a priest to sanctify his hands and feet at the time of service [in the Temple], as [Exodus 30:19] states: "And Aaron and his sons will wash...."
לְקַדֵּשׁ הַכּוֹהֵן יָדָיו וְרַגְלָיו בִּשְׁעַת הָעֲבוֹדָה, שֶׁנֶּאֱמָר: "וְרָחֲצוּ אַהֲרֹן וּבָנָיו..." (שמות ל, יט).
Why · Sefer HaChinuch #106
Sanctifying the hands and the feet at the time of the service
“From the roots of the commandment is the fixed foundation that we have said [that it is] to aggrandize the glory of the [Temple] and all of the [activities] that are done there.”
Who & when
“And it is practiced at the time of the Temple by the males of the priesthood alone.”
Where in the Mishneh Torah
Hilchot Admission into the Sanctuary הלכות ביאת מקדש