Midrasz do Diwrej ha-jamim II 30:27

29) (Vayikra 9:22): "And Aaron lifted his hands to the people and he blessed them (the priestly blessing), and he came down from offering the sin-offering and the burnt-offering and the peace-offerings." The verse is (to be understood as) inverted, (i.e., First "he came down, etc." and then "Aaron lifted his hands, etc."). In going down, he lifted his hands and blessed the people. "and he blessed them": while standing. You say "while standing," but perhaps (it could be done) standing or not standing! It is, therefore, written (Devarim 10:8): "to stand before the L–rd to serve Him and to bless in His name." Blessing is being likened to serving. Just as serving is "standing," so is blessing. And thus is it written (II Divrei Hayamim 30:27): "And the Cohanim and the Levites stood up, and He heard their voice, and their prayer came to His holy abode in heaven." In the generation of Chizkiyahu, king of Judah, when they occupied themselves with Torah, what is written of them? "and their prayer came to His holy abode in heaven." In other generations, when they served idolatry, what is written of them? (Isaiah 1:15): "And when you spread out your hands (in prayer), (I will hide My eyes from you, etc.")