Reading the Haftorah to oneself versus hearing it from the reader:[1]

It is an obligation upon everyone to listen and pay attention to the Haftorah, just as they must listen to the reading of the Parsha by Kerias Hatorah.[2] Accordingly, two people may not read the Haftorah together, out loud, as two simultaneous voices cannot be [properly] heard.[3] [Therefore, ideally the congregation is to remain silent and listen to the reading of the Maftir.] Nonetheless, some are accustomed due to reasons known to them [i.e. Kabbalah], to not rely on the hearing of the Haftorah alone, and rather they read it as well to themselves, and listen to the blessings from the Maftir.[4] Whatever the case, they must be careful not to read the Haftorah aloud but rather quietly.[5] (They are also to be careful to read it word for word with the Maftir.[6]) [Practically, the Chabad custom, when not in the presence of the Rebbe[7], is to read the Haftorah along silently together with the Baal Korei, word by word, in an inaudible voice.[8] However, the custom of many Chassidic communities is to have everyone read it together aloud in the same tone, hence not having any main reader, and have everyone listen to the before and after blessings of the main reader.[9] The custom in Ashkenazi communities who read from a scroll, is for only the Baal Korei to read the Haftorah and everyone else looks long silently, just like by Kerias Hatorah.[10]]

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[1] See Admur 284:11; Ketzos Hashulchan 88:2; Piskeiy Teshuvos 284:11

[2] Admur 284:11; Michaber 146:3 regarding talking; Beis Yosef 146; Rabbeinu Yerucham 2:3 in name of Maharam; Sefer Haminhagim p. 32; See Likkutei Sichos 33:20 [printed in Shulchan Menachem 2:95]

Other opinions: Some Poskim rule one is not obligated to pay attention to the reading of the Haftorah. [Levush 284:1; See Piskeiy Teshuvos 284:1 footnote 1]

[3] Admur ibid; Rama 284:5; Rivash 36; Megillah 21b; M”A 284:5

[4] Admur ibid and 494:6; M”A 284:5 in name of Kesavim; Arizal in Peri Eitz Chaim Shaar Hashabbos 19; Shaar Hamitzvos Parshas Vaeschanon; Aruch Hashulchan 284:14; See Chasam Sofer 68 [brought in Piskeiy Teshuvos 284:5 and 11] that those who read from a printed Chumash [as opposed to a scroll] are required to read along with the reader in order so it not be considered a prohibition of saying the written Torah by heart

[5] Admur ibid and 494:6; M”B 284:11 that if they read aloud, the blessings are in vain; See Shaareiy Efraim 9:33, brought in Biur Halacha 284 “Detrei”, that if everyone says it aloud there isn’t a Minyan listening to the Haftorah and it’s not considered to have been said with a Minyan; The Shaareiy Efraim ibid negates the custom of those who say it aloud; Ketzos Hashulchan 88:2 footnote 7

Other opinions: See coming footnotes regarding the custom of some Chassidic communities

[6] Admur ibid in parentheses; See Admur 183:10; M”B 284:12 in name of P”M

[7] Ketzos Hashulchan Vol. 3 Hearos in back of Sefer for 88:2 that when we are by the Rebbe, we listen to his reading

[8] See Sefer Haminhagim p. 61 [English] “Some are accustomed” and in footnote “Directive of Rebbe in name of the Rebbe Rayatz that so is our custom”; Ketzos Hashulchan 88:2 that so is the custom and in Hearos ibid that only by the Admur do we not read to ourselves

[9] Chasam Sofer 68 that so is the custom in all his provinces and they should be left with their custom; Shaareiy Efraim 9:33 and 10:25 that so is custom in his communities [although he negates it and requests for it to be abolished]; Minchas Yitzchak 3:12; 9:22; See Yachin Boaz 26, brought in Pischeiy Teshuvah 284; Divrei Yatziv 129; Teshuvos Vehanhagos 4:74; Piskeiy Teshuvos ibid;

[10] Maaseh Rav 136, brought in M”B 284:11