הלכה על מגילה 46:18

From the laws of the commandment is that which they, may their memory be blessed, said (Megillah 23b) that the priests only raise their hands (to make the blessing) when there are ten (a prayer quorum), and the priests [are included in] the tally. And how is the raising of the hands? (Sotah 38b, Mishneh Torah, Laws of Prayer and the Priestly Blessing 14:3-4) At the time the prayer leader reaches the [blessing of] service, meaning to say when he wants to begin to say the blessing of Retseh, all the priests in the synagogue go up to the platform [with] their faces towards the ark, their backs towards the people, and their fingers bent upon their palms, until the prayer leader finishes the blessing of thanksgiving. And afterwards, they turn their faces towards the people, extend their fingers and raise their hands across from their shoulders and begin, "May He bless you." And the prayer leader dictates to them word by word, as it is stated, "say to them"; and they repeat after him with a pleasant tone. And when they finish the first verse [of the blessing], all of the people answer, "Amen." And so [too,] with the second verse, and the third. And when they finish [all] three verses, the prayer leader begins "Sim shalom, etc." (the next blessing) and the priests turn their faces towards the holy [ark] and bend their fingers and stand there on the platform until he finishes the blessing of Sim shalom, and [then] return to their places. And our custom today is that the priests do not go up to the platform, but rather stand in front of the ark and follow the [rest of the] order that we have written.