Talmud Yerushalmi Berachot on the Kohanic Blessing

"Rabbi Huna said: Regarding one who is in the presence of the Kohanim when they impart the three Kohanic blessings in the synagoge - when they recite the first blessing, he should say the verse 'Bless YHVH, O His angels, the strong warriors who do His bidding, to obey the voice of His word' (Psalms 103:20). When they recite the second blessing, he should say the verse 'Bless YHVH, all His legions, His servants who do His will' (Psalms 103:21). When they recite the third blessing, he should say the verse 'Bless YHVH, all His works, in all the places of His dominion; bless YHVH, O my soul' (Psalms 103:22).

-- Talmud Yerushalmi, Berachot 5a