Parshas Haazinu is a song, a shirah to Hashem.
Our haftarah is a song as well, shiras Dovid.
This is the general connection between the two. Interestingly, Parshas Haazinu is a short parsha with 52 pesukim, while our haftarah with 51 pesukim, albeit short ones, is perhaps the longest haftarah.
Rav Dovid Feinstein adds another connection, pointing out that Shiras Haazinu was stated and sung by Moshe at the end of his life and the song of Dovid in the haftarah was also stated and sung at the end of his life. Furthermore, Dovid’s song was sung as gratitude to Hashem for saving him from all his enemies and Haazinu ends with Hashem saving Klal Yisrael from all of our enemies.
Our haftarah comes from Shmuel Bais, perek 22, but also appears in Tehillim, perek 18, albeit with some slight differences. The question is: Why are both versions necessary? Wouldn’t one have been enough?
Rav Moshe Eisenman’s book, Music Made in Heaven – Thoughts on Dovid HaMelech and Sefer Tehillim” (pages 47-50), points out that Sefer Shmuel tells us practically nothing of Dovid’s personal life and feelings. It is only in Sefer Tehillim that we meet the true Dovid. The book quotes extensively from Dr. Henry Biberfeld’s David, King of Israel which explains how Dovid’s real life is described in Tehillim:
“This is the true mirror of his being, reflecting every phase of his life. If the historical books (and Tanach) show the outward contours of his image, the Psalms probe his deepest secrets. The Psalms are his world, the world of the idea of G-d with all its variety. It is a world detached from and beyond the material sphere. Hence, allusions to actual events occur only rarely in Psalms. The historical books provide the form to be filled in with idea and experience. We cannot expect such a ‘biography of an idea’ to conform to the usual pattern of history. The life of the idea progresses in a reality above space and time. David’s mind, responsive as no other, found in the omnipresence of G-d the one experience that became the dominant force in his life, the atmosphere and living-space of his existence.
“When the supreme ‘event’ of an idea of G-d was perceived by David, an experience was born of such singular force that it reverberates throughout the ages. The Psalms are the record of that great union, when the impact of the omnipresence of G-d struck the responsive chords of David’s being. David’s reactions to the erratics of life were no longer his own. The Divine in David thought, felt, spoke, and acted through him. The inner history of the life of David, his true biography, was called Tehillim, which we should really loosely translate as ‘Reflections,’ for all of Tehillim are reflections of the Divine in the clear mirror of the soul. Triumphs and tragedies, periods of calm and of unrest, of supreme confidence and hopeless despair, mystic surrender and moral reflections, universal love and implacable wrath, find expression in the Psalms.”
Shiras Dovid, which begins, “Dovid addressed the words of this song to Hashem, on the day that Hashem had saved him from the hand of all of his enemies and from the hand of Shaul,” is understood by the Abarbanel as having been sung by Dovid each time he defeated his enemies. Whenever Hashem saved him, he sang this song of gratitude. It is for this reason that Sefer Shmuel does not attach it to a particular event in Dovid’s life; Dovid sang it after many victories.
Rav Dovid Cohen in Sefer Ohel Dovid cites the Vilna Gaon who says that the mizmor in Sefer Shmuel was written before Dovid sinned with Batsheva and the kapitel in Tehillim was written after the sin. He utilizes this approach to explain the small and subtle differences between the two. With this Vilna Gaon, Rav Cohen suggests why we say “Migdol Yeshu’os Malko” in Shabbos bentching, while during the week we say “Magdil Yeshu’os Malko.” The migdol pasuk comes from the end of the mizmor in Sefer Shmuel while the magdil pasuk is from the end of the mizmor in Tehillim.
On Shabbos, says Rav Cohen, we exist in a world of mei’ein Olam Habah, a microcosm of the next world, a taste of Gan Eden. In this world, there is no sin. This is why we don’t say, “V’Hu rachum yechaper avon, Hashem will atone and forgive our sins” at the beginning of Maariv Friday night because we don’t want to mention sin. So too, we say Migdol in Shabbos bentching because it was written prior to the sin with Batsheva. We say Magdil during the weekday because it comes from the version written after the sin and that is the world we live in during the week.
There are other approaches as well. The Torah Temimah in Baruch She’amar, his sefer on Tefillah, says that our custom is actually the result of a printing error. The original practice was to say Magdil even on Shabbos but the word Migdol was written in parenthesis with the letters shin and beis referring to Shmuel Beis. A later printer assumed that the intention was to say Migdol on Shin Beis, which he thought was a reference to Shabbos, and so printed the Shabbos bentching differently. Rav Chaim Kanievsky (Derech Sichah, page 682) quotes his father, the Steipler Gaon, who met the Torah Temimah and told him he thought it was incorrect because the Avudraham, a Rishon, mentions saying Migdol on Shabbos, and Sefer Shmuel had not yet been broken up into Aleph and Beis during the times of the Rishonim.
The Rogachover Gaon (Shut Tzafnas Panei’ach 2:5) suggests the Migdol change came as a result of a Gemara in Shabbos 115a and 116b where Chazal forbade people from learning Kesuvim on Shabbos, which includes Tehillim. Therefore, the pasuk from Sefer Shmuel, Migdol, is said in bentching and not Magdil from Tehillim. This served as a reminder that after the meal, when people would go learn, they should not learn Kesuvim. (Nowadays, it is permitted.)