TT #27 · Parshas Shemini · Aliyah 2 · Passuk 22
Chabad.org — Vayikra Chapter 9 with Rashi
English: “Aaron lifted his hand”: from here is derived that the priestly blessing is performed with the hands raised.
Sotah 38a.
E-Daf.com — Sotah 38a (Tsurras Hadaf)
Gezeirah shavah — the priestly blessing is identified from the act of Aaron lifting his hands toward the people.
Aaron's raising of his hands is a source for the practice of the priestly blessing with raised hands.