Leviticus 9:22

And Aharon stretched out his hands towards the people and blessed them, and came down from the altar with joy, after he had finished to perform the sin offering and the burnt offering and the hallowed oblation.

spread out his hands toward the people Our ancestors used this verse [Soṭa 38a] to derive exegetically the appearance of the hands during the Priestly Blessing.

he stepped off As I have pointed out in many similar places, this verb is in the past perfect tense. Aaron had already stepped off the altar, having done the people’s sin-offering, and their burnt-offering, and their peace-offering.

Scripture says that he stepped off because the Altar was three cubits high. Moshe and Aaron went from there into the Tent of Assembly — probably to pray for the appearance of the fire — and when they came out, they both blessed the people.

AND AARON LIFTED UP HIS HANDS TOWARD THE PEOPLE, AND BLESSED THEM — “with the priestly benediction: The Eternal bless thee …. The Eternal make His face shine upon thee … The Eternal lift up His countenance upon thee …” Thus the language of Rashi. But if so, the section of Speak unto Aaron and unto his sons, saying: Thus ye shall bless the children of Israel, in the Book of Numbers,, Verse 23. takes chronological precedence over this section! Perhaps it is [indeed] so since it is placed near to that which it says there, And it came to pass on the day that Moses had made an end of setting up the Tabernacle, 7:1. [which was on the first of Nisan — “the eighth day” herein discussed].
It is possible to say that Aaron spread forth his hands towards heaven and blessed the people, just as Solomon did, as it is said, And Solomon stood before the altar of the Eternal in the presence of all the congregation of Israel, and spread forth his hands, toward heaven, and there it is said, And he stood, and blessed all the congregation of Israel with a loud voice, saying, etc., Verse 55. Therefore Scripture does not mention that Moses commanded him to do so [i.e., to say the priestly blessing, since it had in fact not yet been given].
In the Beraitha Note 65. of the section of consecration in the Torath Kohanim I have seen it said: 30. “And he blessed them. This is an unspecified blessing [the nature of which] you do not know. Scripture [therefore] went back and explained it further on: The Eternal bless thee, and keep thee. The Eternal make His face to shine upon thee, and be gracious unto thee. The Eternal lift up His countenance upon thee, and give thee peace.” Nonetheless, I could still say that the Rabbis of this Beraitha were saying as follows: “This blessing with which Aaron blessed the people of his own accord, is unspecified, and Scripture did not explain to us what it was. But the blessing that the priests have been commanded to say in all future generations, has been expressly set forth, it applying to all priests alike forever. Or it may be that [the Rabbis of this Beraitha] are of the opinion that here He commanded him [to say] the priestly blessing for that day, and later on [as stated in the Book of Numbers] this blessing was given to Aaron and his sons for the succeeding generations.”