סימן קז סעיף ג

בַּשַׁבָּת הַגָּדוֹל לְמִנְחָה, נוֹהֲגִין שֶׁאֵין אוֹמְרִים בָּרְכִי נַפְשִׁי, אֶלָּא עֲבָדִים הָיִינוּ וְכוּ'. לְפִי שֶׁבַּשַׁבָּת הַגָּדוֹל הָיְתָה הַתְחָלַת הַגְּאֻלָּה וְהַנִּסִּים (סִימָן ת"ל).

On Shabbos Hagadol¹¹The Shabbos before Pesach is so called because in Egypt, the Jews were commanded to take a lamb on the tenth day of Nisan and designate it for the paschal offering. They did so, and tied the sheep to their bedposts. The Egyptians, upon hearing the Jews were bringing the sheep as a sacrifice, did not protest, even though the lamb was the Egyptian idol. That year, the tenth day of Nisan occurred on Shabbos, and in commemoration of this miracle, we call it Shabbos Hagadol—the Great Shabbos. (Tur 430 etc.) at Minchah it is customary not to recite Barchi Nafshi, but to say instead Avadim Hayinu¹²This is found in the Hagaddah of Pesach. It is customary to recite until Lechappeir al Kol Avonoseinu (to atone all our sins). (Ramah 430) The Vilna Gaon did not agree with this custom, and many people follow his ruling and do not say it. etc., because on Shabbos Hagadol the redemption from Egypt and the miracles began.¹³See note 11.

תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)