Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
First Section – the Positive Commandments
58. It is a positive commandment for Cohanim (priests) to bless the Jewish people
As it states in Bamidbar (Numbers) 6:23-26, "So shall you bless the children of Israel... May Hashem bless you... May Hashem make His face shine upon you... May Hashem lift up His countenance..." If the Cohain adds any other blessing, he has transgressed the prohibition of "Do not add" (Devarim [Deuteronomy] 13:1, see Negative Commandment 159). If a Cohain does not go up to bless the people when the prayer leader says "Cohanim", he has violated a positive commandment and three mitzvos: "So shall you bless", "Say to them", and "So they shall put my name". Any Cohain that blesses will be blessed as it says in Bereshis (Genesis) 12:3, "And I will bless those that bless you."
This applies in all places and at all times.