Positive commandment · Mitzvat Aseh #26 of 613
For the priests to bless the Jews
לְבָרַךְ הַכּוֹהֲנִים אֶת יִשְׂרָאֵל
The verse · Bamidbar 6:23
דַּבֵּר אֶל־אַהֲרֹן וְאֶל־בָּנָיו לֵאמֹר כֹּה תְבָרְכוּ אֶת־בְּנֵי יִשְׂרָאֵל אָמוֹר לָהֶם
“Speak to Aharon and to his sons, saying, In this way you shall bless the children of Yisra᾽el, saying to them,”
The Rambam’s words
For the priests to bless the Jews, as [Numbers 6:23] states: "In this manner, you shall bless the children of Israel."
לְבָרַךְ הַכּוֹהֲנִים אֶת יִשְׂרָאֵל, שֶׁנֶּאֱמָר: "כֹּה תְבָרְכוּ אֶת בְּנֵי יִשְׂרָאֵל" (במדבר ו, כג).
Why · Sefer HaChinuch #378
Priestly blessing every day
“It is from the roots of the commandment that G-d wanted in His great goodness to bless His people through the servants that are always encamped in the House of G-d and all of their thoughts cling to His service and their souls are connected to His fear the whole day.”
Who & when
“And this commandment is practiced every day and at all times by the priests, as this commandment to bless Israel is upon them.”
Where in the Mishneh Torah
Hilchot Prayer and the Priestly Blessing הלכות תפילה וברכת כהנים