Rashi on Numbers 6:23/רש"י על במדבר ו׳:כ״ג

1אמור is infinitive. Similar forms are זכור (Exodus 20:8), שמור (Deuteronomy 5:12); in O. F. disant, (English = saying).

2אמור להם SAYING UNTO THEM — so that all of them should hear the blessing (it would not suffice to pronounce the blessings upon the Israelites in their absence; cf. Sifrei Bamidbar 39).

3אמור is written plene (with ו after the מ) thus indicating: ye shall not bless them hurriedly and hastily, but devoutly and with a whole heart (Midrash Tanchuma, Nasso 10).