Numbers 6:23

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

דַּבֵּ֤ר אֶֽל־אַהֲרֹן֙ וְאֶל־בָּנָ֣יו לֵאמֹ֔ר כֹּ֥ה תְבָרְכ֖וּ אֶת־בְּנֵ֣י יִשְׂרָאֵ֑ל אָמ֖וֹר לָהֶֽם׃ {ס}        

Speak to Aharon and to his sons, saying, In this way you shall bless the children of Yisra᾽el, saying to them,