Kabbalah zu Schemot 18:76

"When Jethro, the priest of Midian, Moses's father-in-law, heard of all that Hashem had done for Moses" (Exodus 18:1). Rabbi Chizkiyah opened the discussion, saying: "And Aaron lifted up his hands" (Leviticus 9:22). "His hands"is spelled without a Yud, thus meaning one hand only. And this is the reason, one should lift up his right hand above his left hand. And we have already explained this secret.