TT #30 · Parshas Shemini · Aliyah 2 · Passuk 22
Chabad.org — English translation + Rashi
English: “He blessed them”: the blessing was performed standing. Just as the Temple service is performed standing, so the blessing is performed standing.
Sifra (Toras Kohanim), Shemini, Mekhilta de-Miluim II 17.
Sefaria — Sifra, Shemini, Mekhilta DeMiluim II 17
Gezeirah shavah — the requirement to stand for blessing is derived by comparison with standing for divine service.
Priestly blessing is performed standing, by comparison with the requirement that priestly service be performed standing.