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Isaiah Horovitz (Hebrew: ישעיה הלוי הורוויץ), (c. 1565 – March 24, 1630), also known as the Shelah ha-Kadosh (the holy Shelah) after the title of his best-known work, was a prominent Levite rabbi and mystic.
Isaiah Horovitz was born in Prague around 1565. His first teacher was his father, Avraham ben Shabtai Sheftel Horowitz, a notable scholar and author, and a disciple of Moses Isserles (Rema). Horowitz studied under Meir Lublin and Joshua Falk. He married Chaya, daughter of Abraham Moul, of Vienna, and was a wealthy and active philanthropist, supporting Torah study, especially in Jerusalem. In 1590, in Lublin, he participated in a meeting of the Council of Four Countries, and his signature is on a decree that condemns the purchase of rabbinic positions. In 1602, was appointed head of Beis Din in Austria, and in 1606 was appointed Rabbi of Frankfurt am Main. In 1614, after serving as rabbi in prominent cities in Europe, he left Frankfurt - following the Fettmilch uprising - and assumed the prestigious position of chief rabbi of Prague. In 1621, after the death of his wife, he moved to Palestine, was appointed rabbi of the Ashkenazic community in Jerusalem, and married Chava, daughter of R. Eleazer. In 1625, he was kidnapped and imprisoned, together with 15 other Jewish rabbis and scholars, by the Pasha (Ibn Faruh) and held for ransom. After 1626, Horowitz moved to Safed, erstwhile home of Kabbalah, and later died in Tiberias on March 24, 1630 (Nissan 11, 5390 on the Hebrew calendar).
In his many Kabbalistic, homiletic and halachic works, he stressed the joy in every action, and how one should convert the evil inclination into good, two concepts that influenced Jewish thought through to the eighteenth-century, and greatly influenced the development of the Chassidic movement.
Famous descendants of Isaiah Horowitz included Yaakov Yitzchak of Lublin, (Yaakov Yitzchak Horowitz; Hebrew: יעקב יצחק הורוביץ), known as "The Chozeh of Lublin" (Hebrew: החוזה מלובלין, The Seer of Lublin), the prominent Billiczer Rabbinical family of Szerencs, Hungary and the Dym family of Rabbis and communal leaders in Galicia.
His most important work Shnei Luchos ha-Bris (Hebrew: שני לוחות הברית) (Two Tablets of the Covenant); abbreviated Shelah של"ה, is an encyclopedic compilation of ritual, ethics, and mysticism. It was originally intended as an ethical will - written as a compendium of the Jewish religion. The title page of the first edition states that the work is "compiled from both Torahs, Written and Oral, handed down from Sinai". The work has had a profound influence on Jewish life - notably, on the early Hassidic movement, including the Baal Shem Tov; Rabbi Shneur Zalman of Liadi was described as a "Shelah Yid", and Shelah clearly echoes in Tanya. The Shelah has been often reprinted, especially in an abbreviated form. The work was first published in 1648 by his son, Rabbi Shabbethai Horowitz.
Tefillat HaShlah - The Shelah's Prayer 
Rabbi Horowitz wrote that the eve of the first day of the Hebrew month of Sivan is the most auspicious time to pray for the physical and spiritual welfare of one's children and grandchildren, since Sivan was the month that the Torah was given to the Jewish people. He composed a special prayer to be said on this day, known as the Tefillat HaShlah - the Shelah's Prayer. In modern times, the custom of saying this prayer on the appointed day has become very popular among Orthodox Jewish parents.
- Horowitz, Isaiah, jewishencyclopedia.com
- Short biography of Rabbi Isaiah Halevi Horowitz (The Shelah), chabad.org
- Rabbi Isaiah ben Avraham Ha-Levi Horowitz (The Shelah), ou.org
- Rabbi Isaiah ben Avraham Ha-Levi Horowitz (The Shelah), jewishvirtuallibrary.org
- Rabbi Isaiah HaLevi Horowitz –The Shelah, meaningfullife.com
- Rabbi Yishayahu ben Avraham Ha-Levi Horowitz, horwitzfam.org
- Shney Luchot Habrit, urimpublications.com
- Tefillat HaShlah Custom in Modern Life, mavenmall.com
- , famroots.org
- , meaningfullife.com
- , hakolhayehudi.co.il
- Shney Luchot Habrit: fulltext download from this site (Hebrew)
- "Shney Luchot Habrit", Translator Rabbi Eliyahu Munk, Urim Publications 2000. ISBN 965-7108-07-1
- Isaiah Horowitz: The Generations of Adam. Ed. by Miles Krassen. New York 1996.
- Text of Tefillat HaShlah - Hebrew
- "Life and teachings of Isaiah Horowitz", Rabbi Dr. E. Newman, Judaica Press 1972. ISBN 0-9502739-0-2