TT #28 · Parshas Shemini · Aliyah 2 · Passuk 22
Chabad.org — Vayikra Chapter 9 with Rashi
English: “His hand” is written in the singular: from here the Sages derive that the priest must raise his right hand slightly higher than his left during the priestly blessing.
Hagahos Maimoniyos, Hilchos Tefillah 14:3.
English: “His hand” is written in the singular. From here it is derived that the priest must raise his right hand slightly higher than his left when giving the priestly blessing.
Textual inference — the singular written form of “his hand” is used to derive the positioning of the hands in priestly blessing.
The ketiv singular form is used as a basis for positioning the right hand slightly higher than the left during priestly blessing.