וְדִבַּרְתָּ֤ אֲלֵיהֶם֙ אֶת־כָּל־הַדְּבָרִ֣ים הָאֵ֔לֶּה וְלֹ֥א יִשְׁמְע֖וּ אֵלֶ֑יךָ וְקָרָ֥אתָ אֲלֵיהֶ֖ם וְלֹ֥א יַעֲנֽוּכָה׃
Und du sollst alle diese Worte zu ihnen reden, aber sie werden nicht auf dich hören; du sollst auch zu ihnen rufen, aber sie werden dir nicht antworten.
Shulchan Arukh, Orach Chayim
1. The Laws Pertaining to the Haftarah and Its Blessings, 7 Seifim: We conclude in the Prophets based upon the subject of the parashah, and we do not decrease [the length of the haftarah] from 21 verses other than if the matter is settled in less than that, for example, “add your burnt offerings to your sacrifices” [in Jeremiah 7:21]. RAMA: And only on Shabbat on the matter of 21 verses: three verses for each one of the [seven Torah] readings, but on a Festival on which five read [the Torah], fifteen verses suffice (Maharam). And it was decreed that the haftarah is only [read] in public after they read the Torah (Ramban’s Responsa, Siman 199), but nevertheless it is forbidden to read with the blessings before it and after it; but with no blessing it is permitted (Beit Yosef).